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Geological setting, nature of mineralisation,
and fluid characteristics of the Wang Yai
prospects, central Thailand
John Vernon de Little
BSc
A Research Thesis submitted in partial fulfillment, of the requirements of
the Degree of Bachelor of Science with Honours
School of Earth Sciences,
CODES Centre of Excellence
University of Tasmania November 2005
i
Declaration
This thesis contains no material which has been accepted for the award of any other degree or diploma in any tertiary institution, and to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference is made in the text of the thesis.
Signed
John Vernon de Little
November 2005
ii
Abstract The Wang Yai prospects are located along the Loei Foldbelt, central Thailand. They
are a newly discovered resource and up until now have not been the subject of any
scientific research. Eleven vein systems at Wang Yai were investigated in terms of
vein textures, mineralogy, fluid inclusions and stable isotopes. The vein systems
occur as either well-preserved outcrop or sparse sporadic float. The vein systems can
be divided into two types. They include: (1.) mineralised veins which are
characterised by quartz - chalcedony - calcite ± adularia and (2.) poorly mineralised
veins characterised by chalcedony and opaline quartz. The vein systems are hosted in
volcanic and volcanosedimentary units intruded by andesite and diorite. LA-ICP-MS
U-Pb zircon geochronological studies have revealed the age of host rocks as Late
Permian to Early Triassic.
Volcanology studies of the host rocks indicate that the depositional environment was
a submarine below wave base setting. Zr/Y vs Zr plot of least altered volcanic rocks
have identified the andesitic sandstone and andesitic lithic breccia as having island arc
affinities. The younger andesite and diorite intrusives were found to have continental
arc affinities. These findings are consistent with interpretations of the volcanic setting
at the nearby low-sulphidation Chatree deposit.
Vein texture, mineralogy, alteration and paragenesis studies have identified zonation
between gold-bearing vein systems and barren vein systems. Gold-bearing systems
yield crystalline quartz, lattice-bladed quartz, pseudo-acicular quartz and
pseudomorphs after adularia. Ore mineralogy in these systems is hosted in ginguro
bands which comprise of electrum ± argentite ± pyrite assemblages. Barren vein
systems yield chalcedony and opaline quartz assemblages with associated
recrystallisation textures. Based on quartz textural analysis and ore mineralogy
assemblages the gold-bearing veins are interpreted to have formed at levels deeper in
the system than the barren vein systems.
Fluid inclusion studies of quartz indicate salinities are between 1 to 5 wt % NaCl
equiv. Fluid inclusion homogenisation data yields three different ranges. Crystalline
gold-bearing quartz yields temperatures of 187 – 218 C˚, comb quartz yields
temperatures of 260 - 290 C˚.and crustiform-colloform veins yield temperatures of
140 – 200 C˚.
iii
Stable sulphur isotope studies of pyrite range between +3.82 and -0.04 per mil. These
values indicate a magmatic source. Stable oxygen isotope values of quartz range from
+11 to +17.5 per mil. Low oxygen isotope values range from +11.0 to +13.0 per mil
and are associated with crystalline quartz gold bearing systems. High oxygen isotope
values (+15.0 to +17.5 per mil) are associated with barren quartz chalcedony
dominated systems.
The volcanic setting, vein textures, mineralogy and alteration assemblages of the
Wang Yai prospects are comparable with the nearby low-sulphidation epithermal
Chatree deposit. The Wang Yai prospects appear to be at the top of the broader
epithermal system and therefore potential for high grade mineralisation at depth
exists.
iv
Acknowledgements I would firstly like to thank my supervisor, Dr Khin Zaw for granting me the opportunity to
undertake this fun and interesting project. Khin Zaw’s enthusiasm and great expertise in the
area have been a motivating factor throughout the year. Khin has provided me with all the
logistical support needed in order for things to run smoothly. I also owe a great deal of
gratitude to my co-supervisors, Drs. Stuart Smith and Anthony Harris. Stuart played an
invaluable role early in the year during my time of field mapping. His excellent introduction
to the area and help during the first week in Thailand put me in good stead for the remaining 4
weeks. Anthony has provided me with many new ideas, and feedback. I would especially
like to thank Anthony for his time and patience during the final two weeks of the year. A
special thank you goes out to Sebastian Meffre who provided his expertise during field
mapping, dating of host rocks at Wang Yai, and driving the microprobe.
Secondly I would like to thank the Thai Global Ventures Co., Ltd exploration team. Thank
you Ian Cameron for making me feel welcome and supporting me during my time in
Thailand. Henry Agupitan - thanks for your hospitality at Wan Hin and help with mapping
and organisation of samples sent to me. Lek, Wittaya, Kittipong, C, Bank and Trent - thank
you for taking me under your wing and making my time in Thailand enjoyable. I would also
like to thank Pen for serving delicious Thai meals three times a day during 12 hour mapping
excursions.
Many thanks also go out to the staff at University of Tasmania: Simon Stephens - thank you
for supplying me with my constant need of more thin sections, Jim Hutton - your expertise
with the microprobe instrument was invaluable, Keith Harris - thank you for undertaking
sulphur isotope analysis at such late notice and Phil Robinson and Katie McGoldrick for
XRF analysis.
Last but not least I would like to thank Laijing and my family and friends. Laijing has
supplied me with support and motivation through very trying times and has put up with my
crankiness. I would also like to thank her family for shipping strong Malaysian Ipo ‘White
elephant’ coffee to me. Dad and Mum, thankyou for your support and invitations to dinner
during the final weeks of the year. To my fellow honours students McGeisha, Sekins
(cAmeRa), Corzaaaa, Kit-Kenobi, Kingy (still has difficulty playing my spin bowling), Kat, Mel, and
Kim thank you for your help, support and fun times during the year.
v
TABLE OF CONTENTS List of Figures viii
List of Tables xi
CHAPTER 1: INTRODUCTION 1 1.1 PREAMBLE 1
1.2 LOCATION 1
1.3 ACCESS 3
1.4 TOPOGRAPHY 4
1.5 LAND USE 4
1.6 EXPOSURE 5
1.7 EXPLORATION HISTORY 7
1.8 AIMS 7
1.9 METHODOLOGY 8
CHAPTER 2: REGIONAL GEOLOGY 10 2.1 TECTONIC EVOLUTION 10
2.2 LOEI FOLDBELT 11
2.3 DISTRICT SCALE GEOLOGY AT WANG YAI 13
2.4 GEOCHRONOLOGY OF THE LOEI FOLDBELT 16
2.5 AGE OF HYDROTHERMAL ALTERATION 18
CHAPTER 3: LOCAL GEOLOGY 20 3.1 INTRODUCTION 20
3.2 STRATIGRAPHY OF WANG YAI 21
3.3 NON-VOLCANOGENIC SEDIMENTARY ROCKS 24
3.3.1 Fossiliferous limestone 24
3.3.2 Siltstone 25
3.4 VOLCANIC SUCCESSION 27
3.4.1 Andesitic volcaniclastic sandstone 27
3.4.2 Polymict andesitic lithic breccia 28
3.4.3 Polymictic massive volcanic conglomerate 30
3.4.4 Quartz polymictic breccia 33
3.5 COHERENT ROCKS 34
3.5.1 Quartz phyric rhyodacite 35
3.5.2 Feldspar phyric andesite 36
vi
3.5.3 Feldspar phyric diorite 38
3.5.4 Quartz diorite 39
3.6 WHOLE ROCK GEOCHEMISTRY 41
3.7 GEOCHEMICAL CLASSIFICATION OF WANG YAI 42
3.8 SUMMARY 43
3.8.1 Environment of deposition 44
3.8.2 Sequence of events 44
3.9 COMPARISON OF WANG YAI AND CHATREE GEOLOGY 44
CHAPTER 4: GOLD-BEARING QUARTZ VEINS 51 4.1 INTRODUCTION 51
4.2 CONICAL HILL 49
4.2.1 Quartz vein textures 51
4.3.1 Conical Hill (West) 51
4.3.2 Ore mineralogy 54
4.4 CENTRAL RIDGE 58
4.4.1 Quartz vein textures 60
4.4.2 Ore mineralogy 61
4.5 T1 HILL 63
4.5.1 Quartz vein textures 65
4.5.2 Detailed vein and alteration assemblage 67
4.5.2.1 Silicic alteration assemblages 67
4.5.2.2 K-feldspar alteration assemblages 68
4.5.2.3 Argillic and phyllic alteration assemblages 69
4.5.2.4 Chloritic alteration assemblages 69
4.5.2.5 Propylitic alteration assemblages 70
4.5.3 Ore mineralogy 70
4.6 T4 HILL 72
4.7 GIFT PROSPECT 72
4.7.1 Quartz textures 74
4.7.2 Alteration 78
4.7.3 Ore mineralogy 78
4.8 MINERALOGY AND TEXTURAL ZONING 79
4.9 INTERPRETATION 83
vii
CHAPTER 5: FLUID INCLUSIONS 91 5.1 OXYGEN ISOTOPES 91
5.1.1 Results 93
5.2 SULPHUR ISTOPES 97
5.3 FLUID INCLUSION STUDIES 98
5.3.2 Homogenisation temperatures 99
5.3.2 Salinity 100
5.4 DISCUSSION 101
5.4.1 Oxygen isotopes 101
5.4.2 Sulphur isotopes 103
5.4.3 Fluid inclusions 104
CHAPTER 6: CONCLUSION 106
6.1 Geological setting 106
6.2 Nature of Mineralisation 106
6.3 Fluid characteristics 107
6.4 Significant findings and implications for exploration 108
REFERENCES 109
APPENDICES
1. LITERATURE REVIEW
2. MICROPROBE DATA
3. XRF DATA
4. STABLE ISOTOPE DATA
viii
LIST OF FIGURES 1.1 Map of showing the location of the Wang Yai prospects and the neighboring
epithermal low-sulphidation Chatree deposit. 2
1.2 Map showing the location of the Wang Yai prospects, Chatree Mine, major
provinces and districts. 3
1.3 (A-B) Photographs of land use at Wang Yai 5
1.4. Aerial photograph of the Wang Yai tenement with location of major hills,
prospects, and the town of Wan Hin. 6
2.1. Distribution of continental terranes and sutures within and adjacent to Thailand.
12
2.2. Map showing the Pitchit-Petchabun district scale geology and location of Wang
Yai study area and Chatree gold mine. 14
2.3. Map showing the district scale geology of the Wang Yai district. 15
2.4. LA-ICP-MS U-Pb Concordia plot – Quartz phyric rhyolite. 17
2.5. LA-ICP-MS U-Pb Concordia plot – Quartz phyric sandstone/breccia 18
3.1. Geological map of south Wang Yai showing the location of lithological units and
distribution of vein float, subcrop, and stockwork. 22
3.2. Geological map of the Gift Prospect showing the location of lithological units and
distribution of vein float, subcrop, and stockwork. 23
3.3. (A-E) Photographs showing petrological and textural characteristics of
fossiliferous limestone and siltstone. 26
3.4. (A-B) Photographs showing accretionary lapilli and textural characteristics of
andesitic sandstone. 28
3.5. (A-D) Photographs showing petrological and textural characteristics of massive
volcanic conglomerate. 32
3.6. (A-B) Photographs showing petrological and textural characteristics of quartz
polymictic breccia. 34
3.7. (A-B) Photographs showing petrological and textural characteristics of quartz
phyric rhyodacite. 36
3.8. (A-B) Photographs showing petrological and textural characteristics of feldspar
phyric andesite. 37
3.9. (A-B) Photographs showing petrological and textural characteristics of fine
grained perlitic breccia. 38
ix
3.10. (A.B) Photographs showing petrological and textural characteristics of feldspar
phyric diorite. 39
3.11. (A-B) Photographs showing petrological characteristics of quartz diorite. 40
3.12. Zr/TiO2 and Nb/Y rock discrimination diagram of andesitic sandstone, andesitic
lithic breccia, rhyodacite, quartz diorite, and feldspar phyric andesite. 42
3.13. Zr/Y vs Zr discrimination diagram of andesitic sandstone, andesitic lithic
breccia, quartz diorite, rhyodacite, and feldspar phyric andesite. 43
4.1. Aerial photo of the Wang Yai tenement showing the locations and names of vein
systems. 48
4.2. Fact map of Conical Hill showing the location of subcrop for the main high grade
vein system and location of vein float for the second system to the west. 50
4.3. (A-F) Photographs showing quartz textural characteristics of Conical Hill. 53
4.4. (A-F) Photomicrographs showing the ore mineralogy characteristics of Conical
Hill. 56
4.5. Frequency histogram of gold fineness values for Conical Hill. 57
4.6. Photograph of Central Ridge. 58
4.7. Fact map of Central Ridge vein system. 59
4.8. (A-B) Photomicrographs of ore mineralogy at Central Ridge. 61
4.9 (A-F) Photographs showing the quartz textures at Central Ridge. 62
4.10. Photograph of T1 Hill. 63
4.11. Fact map showing the location of the main vein system at T1 Hill. 64
4.12. (A-F) Photographs showing the quartz textures at T1 Hill. 66
4.13. Photograph showing K-Feldspar alteration. 69
4.14. (A-B) Photomicrographs showing textures and characteristics of ore
mineralogy at T1 Hill. 71
4.15. Fact map of Gift prospect showing the location of vein systems, vein float and
surface alteration. 73
4.16. (A-B) Photographs showing the textural characteristics of vein system ‘F’ in the
Gift prospect. 76
4.17. (A-F) Photographs showing the textural characteristics of A, B, C, D, E vein
systems in the Gift prospect. 77
4.18. (A-B) Photomicrographs showing the characteristics of ore mineralogy at Gift
prospect. 79
x
4.19. Epithermal model developed by Buchanan (1981) with the Wang Yai vein
systems superimposed. 84
5.1. Aerial photo of Wang Yai show the raw � 18� SMOW values in per mil for each
Gift prospect vein system. 94
5.2. Frequency histogram of calculated � 18� SMOW values of ore fluids at 180ºC,
250ºC, and 300ºC. 96
5.3. Frequency histogram of � 34S values for wall rock and vein located pyrite. 97
5.4. Photomicrographs showing characteristics of Type I and II fluid inclusions. 99
xi
List of Tables 2.1. Summary of the characteristics of magmatic events in the Loei Folbelt, Thailand.
16
4.1. Summary of microprobe analyses of argentite at Conical Hill. 55
4.2. Summary of microprobe analyses of electrum at Conical Hill. 57
4.3. Summary of primary, recrystallisation, and replacement quartz textures for
Conical Hill, Central Ridge, T1 Hill, Gift prospect and T4 Hill. 81
5.1. Summary of � 18� SMOW values for quartz at Wang Yai. 92
5.2. Summary of isotopic compositions of ore fluids for calculated temperatures. 95
5.3. Summary of � 34S values for vein and wallrock located pyrite. 96
5.4. Summary of fluid inclusion data, Wang Yai. 101
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 1 Introduction
1.1 Preamble
Epithermal gold mineralisation at the Wang Yai prospects, central Thailand is a resource
which has been know for over 10 years and up until now no scientific research of the
rocks have been undertaken. The Wang Yai district is being actively explored by Thai
Global Ventures Co., Ltd. Exploration to-date includes soil and rock chip sampling
combined with limited soil geochemistry, RAB, RC and diamond drilling. The results
from this preliminary exploration have highlighted the need for further investigation.
This thesis will build on the exploration data sets already available and provide new
information from the field work undertaken in February 2005. Particular emphasis is
given to the well preserved outcrops of epithermal veins in the district. Analytical
techniques such as vein texture studies, petrography, stable oxygen and sulphur isotopes,
and fluid inclusions were used to help better constrain the physiochemical character of
fluids that were associated with the formation of the Wang Yai prospects.
1.2 Location
Wang Yai prospects are located in the Petchabun Province, central Thailand
approximately 300km north of Bangkok and 15km NE of the Chatree gold mine (Figure
1.1). The small town of Wan Hin lies on the eastern border of the tenement and is the
site of the Erawan Mining camp and accommodation for employees. Wan Hin is
approximately 5km northwest of Wang Pong (Figure. 1.2). The Wang Yai tenement
covers an area of 3 by 2km and consists of three different prospects. The prospects
include Gift (in the north) , S.V (to the east), and the T4 Hill, T1 Hill, Central Ridge, and
Conical Hill (in the south; Figure 1.4). In this study the Gift prospect and southern
prospects (T4 Hill, T1 Hill, Central Ridge, and Conical Hill) will be the main focus.
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 1.1 Map of Thailand showing the location of the Wang Yai prospect and the neighbouring epithermal low-sulphidation Chatree deposit to the south east
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
1.3 Access
Access to the Wang Yai prospects is via a series of sealed bitumen roads linking the
major provinces and towns. Within the tenement, gravel roads and tracks provide
excellent access throughout the dry season to all parts of the prospects. During the wet
season the roads become inaccessible to light vehicles and restricted to 4WD vehicles.
Figure 1.2 Map showing the location of the Wang Yai prospects, Chatree Mine, major provinces and districts (modified after Kromkhun, 2005).
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
1.4 Topography
The topography of the area consists of relatively flat alluvial plains and a series of north
south trending, low hills (up to 160m). In the south of the tenement, 60% of the land is
dominated by 5 major hills and gently undulating topography. The remainder constitutes
the top north western corner which is dominated by flat low lying alluvial deposits. The
major hills and their corresponding heights in the Wang Yai prospect include Conical
Hill (140m), Central Ridge (155m), T1 Hill (125m), and T4 Hill (135m). These hills are
characterised by the occurrence of well preserved north south trending quartz vein float
and silica hematite altered volcanogenic host rocks. Outcrop is sparse. Dividing the
elevated land from the low lying land is the Khlong Namman River which meanders
through the prospect in a south west direction. The associated alluvial deposits occur on
the northern side of the river and extent up to 1km away from the river and into the south
of Gift prospect. The alluvium covers outcrop. Land on the southern side of river is
more elevated and as a result the lateral extent of alluvial deposition is less.
1.5 Land use
Land use in the area is dominated by agricultural practices (>80%) such as teak, tamarind
and eucalyptus plantations, rice and corn farming. Farming practices, with the exception
of rice, are generally located on the flanks of hills as soils are thinner and provide
excellent drainage. Rice farming is restricted to fertile low-lying land which is subject to
flooding during the wet season (Fig. 1.3A). As a result of the extensive agriculture most
of the natural vegetation has been cleared. Minor pockets still exist in areas which are
not suitable for farming such as some of the steep hill tops in the Gift prospect. The
climate in the area consists of three seasons: summer (March to May), a rainy season
(June to October), and winter (November to February).
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
1.6 Exposure
Exposure in the Wang Yai tenement is variable in and between the prospects. The
elevated areas (Conical Hill, Central Ridge, and T1 Hill) of the Wang Yai tenement
provide good outcrop of vein systems and the volcanic host rocks. Outcrop in the Gift
prospect is less common and is mainly restricted to the hill tops, dam walls and irrigation
trenches. Throughout the entire tenement exposure is increased during the dry season
when farmers undergo routine burn offs. Burn offs were undertaken before and during
the field work in February 2005 which resulted in excellent exposure of float and the
accessibility needed to construct accurate geological maps despite the lack of outcrop.
Figure 1.3. A) Photo showing farming on flat low lying land and corn plantations on the hill flanks, taken from T1 Hill looking south east, (photo pers. Comm. Stuart Smith). B) Photo looking north west from Kham Hill in the Gift prospect showing the typical mapping terrain.
A
B
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 1.4. Aerial photograph of the Wang Yai tenement with location of major hills, prospects, and the small town of Wan Hin, central Thailand
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
1.7 Exploration History
In the 1990’s the Special Prospecting Licenses for Wang Yai were granted to Phelps
Dodge Ltd. Exploration in the area was undertaken by Erawan Mining Ltd which was in
joint venture with Phelps Dodge at the time. Erawan Mining Ltd conducted magnetic
surveys, radiometrics, AEM, RC drilling, trenching and diamond drilling at T1 Hill. In
the year 2000 the exploration licences held by Phelps Dodge Ltd expired and were not
renewed. Thai Global Ventures Co., Ltd (which is owned by Thai Goldfields NL, in joint
venture with Oxiana Ltd) then applied for, and was granted new Special Prospecting
Licenses for Wang Yai in the year 2000. Thai Global Ventures Co., Ltd has conducted
work on the area from 2000 to present. This work has included rock chip collection,
trenching, RC drilling and three additional diamond drill holes at T1 Hill. A further 2
diamond drill holes at Conical Hill are planned and this will commence once tenement
licences are granted (pers.comm., Ian Cameron).
1.8 Aims
This study has several major aims, they include the following:
• To provide an understanding of the geological setting of the epithermal systems in the Wang Yai area.
• To describe and interpret the nature of mineralisation and alteration in the known prospect areas.
• To conduct a preliminary study of fluids responsible for the mineralisation in the area.
• To compare and contrast the Wang Yai systems with current models for epithermal systems with particular emphasis given to the nearby Chatree epithermal gold deposit.
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
1.9 Methodology
The methods used in this study include; geological mapping, logging of diamond drill
core, transmitted and reflected light petrological examination, macroscopic and
microscopic examination of quartz vein textures, stable isotope analyses, and fluid
inclusions.
Geological mapping and logging of diamond drill was commenced at theWang Yai
district Thailand on 2nd February 2005. The aims of the geological mapping were to
construct accurate 1:2500 base maps detailing the location of geology, alteration, and
vein systems. The mapping was undertaken by foot with the aid of a 4WD for
transportation to and from the prospect and also to areas that were difficult to access. A
total of nine 1:2500 maps were constructed for the southern area of the Wang Yai
prospects and 12 maps for the Gift prospect. During the mapping representative samples
of the geology and veins were collected for detailed reflected and transmitted light
examination. Nine diamond drill holes from T1 Hill were logged and five were used to
construct a representative section. Field work was completed by 6th March 2005.
Detailed petrographic analysis and volcanologic analysis was conducted on each
lithofacies to determine the depositional processes and environment, in order to compare
rocks at Wang Yai with the stratigraphy at the Chatree gold mine. Geochemical
characteristics of each lithofacies were determined by X-ray Fluorescence, (XRF) at the
University of Tasmania (methods in Appendix 2).
Geochronology was undertaken using zircon LA ICPMS U-Pb on 5 samples. K-Ar was
also used to constrain the timing of hydrothermal mineralisation (methods in Appendix
3).
A macroscopic and microscopic examination was used to describe the characteristics of
vein textures. Microprobe analysis was conducted on sulphide grains to determine their
composition, and highlight trace element variations.
Chapter 1. Introduction
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Stable oxygen isotope analysis of 16 quartz vein samples was undertaken to infer the
fluid source and/or important ore forming processes. Oxygen isotope data of vein quartz
from each vein systems was used to establish a correlation between gold grade and
oxygen isotope values (as to assess its usefulness as a vector to higher grade portions of
the veins). Sulphur isotopes analysis of vein and wall rock pyrite were used to constrain
the source of sulphur. Limited fluid inclusion microthermometry of vein quartz was used
to determine the temperature and salinity of fluids and confirm inferences made for
observed mineralogy and textures at Wang Yai.
Chapter 2 Regional Geology
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 2 Regional Geology
2.1 Tectonic Evolution
Thailand comprises of two main tectonic terranes that include the Shan-Thai Terrane
(west) and the Indochina Terrane (east). These two terranes are bound together along the
north south trending Nan Suture zone (Figure 2.1). The Shan-Thai Terrane, lies on the
western side of the Nan Suture, underlies the western and peninsular of Thailand, eastern
Myanmar, southwestern Yunnan, and the western Malay peninsular (Mecalfe, 1996).
The geology comprises of Pre Cambrian basement rocks (gneiss, marble, calc-silicates,
schists and quartz) overlain by Cambrian and Ordovician silicalastics, carbonates, and
Permian shallow marine sequences (Charusiri et al., 2002). Paleogeographical
reconstruction show that Shan-Thai Terrane was once connected to, the northwestern
Australian Gondwanaland margin (Charusiri et al., 2002). During the Lower Palaeozoic,
the Shan-Thai Terrane rifted off the Indian-Australian margin (Metcalfe, 1996). This
separation from Gondwanaland coincided with a marine transgression synchronous with
the formation of Palaeotethys (Charusiri et al., 2002). The Shan-Thai Terrane migrated
northwards towards the paleoequator and eventually collided with the Indochina Terrane
block during the Permian-Triassic (Charusiri et al., 2002).
The Indochina Terrane flanks the eastern side of the Nan-Uttaradit-Sra Kaeo Suture (see
Figure 2.1) and underlies northern Thailand, Cambodia, most of Lao PDR, Vietnam, and
eastern Malaysia (Figure 2.1; Charusiri et al., 2002). The geology of the Indochina
Terrane consists of Precambrian basement rocks overlain by Palaeozoic shallow marine
sequences and Mesozoic continental deposits (Sashida and Igo, 1999). The Indochina
Terrane was derived from the northern and north eastern margin of Gondwanaland
(Burrett et al., 1990). It began to rifted away from Gondwanaland during the Devonian to
Early Carboniferous (Metcalfe, 1996). During the early Triassic the Indochina Terrane
collided with Shan-Thai, which resulted in the closer of Palaeotethys. Closure and
Chapter 2 Regional Geology
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
sitching of these terranes occurred along the Nan Suture (Charusiri et al., 2002).
Emplacement of I-type and S-type granites, development of NE and NW trending faults,
uplift and erosion followed the collision of the two terranes.
2.2 Loei Foldbelt
The Loei Foldbelt is located in central Thailand on the western margin of the Indochina
Terrane (Figure 2.1). Wang Yai is located in this foldbelt. The Loei Foldbelt trends
north south and is bound with the Sukothai Foldbelt in the south by the Chao-Phraya
strike slip fault (Sitthithaworn and Wasuwanich 1992). According to Sitthithaworn and
Wasuwanich (1992), the depositional environment of the Loei Foldbelt represents a
remanent island arc which occurred was the result of western-ward subduction of the
leading edge of the Indochina Terrane.
The geology of the Loei Foldbelt comprises of units ranging in age from the Ordovician
through to the Mesozoic (Intasopa, 1993). Geochemical evidence suggests that
Precambrian crystalline basements rocks occur beneath the Loei Foldbelt, however they
do not outcrop (Intasopa, 1993). Rocks of the Silurian-Devonian period consist of
volcanic arc andesitic and rhyolitic suites and their volcaniclastic equivalents (Intasopa,
1993). Sedimentary units such as shale, siliceous carbonaceous shale, and chert also
occur (Intasopa, 1993). The Carboniferous period represents the syn-tectonic deposition
of chert, siliceous shale, tuffaceous sandstone, sandstone, conglomerate and limestone
from an environment that changed from deep marine during the Early Carboniferous to a
coastal swamp environment during the Late Carboniferous (Harris et al., 2004). This
period is also marked by island arc backarc volcanism associated with the deposition of
tholeiitic basalts (Intasopa, 1993).
During the Triassic period large granitoids together with rhyolitic volcanic and
volcaniclastics were emplaced along the Loei Foldbelt (Harris et al., 2004). Associated
Chapter 2 Regional Geology
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
with this magmatism is significant Cu and Au mineralisation along the length of the Loei
Foldbelt. This includes the multi million once Chatree Au-Ag epithermal deposit.
Figure 2.1 Distribution of continental terranes and sutures within and adjacent to Thailand
(modified from Harris et al., 2004)
Chapter 2 Regional Geology
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Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
The Jurassic to Cretaceous period marks the uplifting event which subsequently resulted
in the erosion and deposition of the continental red-bed sequences know as the Khorat
Group (Charusiri et al., 2002).
2.3 District scale geology of Wang Yai
Wang Yai is located along the Loei Foldbelt in the Pitchit – Petchabun district. The
district scale geology of Wang Yai comprises of Carboniferous, Permian – Triassic and
Jurassic – Cretaceous Formations (Figure 2.2). Carboniferous rocks include the Huai Hin
Lat and Dok Du Formations and consist of cherts, siliceous shale, tuffaceous sandstone,
sandstone, conglomerate and limestone. Permo-Triassic rocks in the Wang Yai district
include suites of rhyolite, dacite, andesite, basaltic andesite and their volcaniclastic
equivalents (Figure 2.3; Diemar and Diemar 1999). Wang Yai quartz veins and the
nearby Chatree gold deposit, are hosted in these Permo-Triassic rocks (Figure 2.3).
Jurassic – Cretaceous continental red beds of the Khorat Group occur to the north of
Wang Yai and form thick sequences of intra-continental derived sediments which
unconformably overlie Permian – Triassic volcanics. Much of the district scale geology
is covered by thick Quarternary slope wash, valley plain, and residual deposits. These
deposits occur as a thick cover over the older volcanosedimentary Permian – Triassic
successions (Figures 2.2 and 2.3).
Structure in the region comprises of regional north-south, northwest and northeast
faulting which has been interpreted to have initiated from the Triassic orogenesis (Diemar
and Diemar 1999).
Chapter 2 Regional Geology
- 14 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 2.2. Map showing the Pitchit – Petchabun district scale geology and location of Wang Yai study area and Chatree gold mine (modified after Khin Zaw, 2005).
Chapter 2 Regional Geology
- 15 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 2.3 Map showing the district scale geology of the Wang Yai district. The location of Wang Yai study area and Chatree gold mine are labeled (modified from DMR map (1993).
Chapter 2 Regional Geology
- 16 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
2.4 Geochronology of the Loei Foldbelt
Geochronological studies of the Loei Foldbelt revealed five main events of magmatism
occurred between the Ordovician and Triassic periods (Intasopa, 1993; Khin Zaw et al.,
1999; Harris et al., 2004). They include Silurian (435-410 Ma), Devonian (400-350 Ma),
Carboniferous (350 Ma), Early Permian (300-275 Ma), Triassic (245 – 210 Ma) and
Tertiary events (Intasopa, 1993). The characteristics of magmatism events are shown in
Table 2.1. Data from this table was obtained from Harris et al. (2004) ‘epochs of
magmatism’ which includes a summary of previous literature together with new LA-ICP-
MS U-Pb zircon and 40Ar/39Ar geochronology as part of the AMIRA P390A project.
Table 2.1 Summary of the characteristics of magmatic events in the Loei Foldbelt, Thailand
Event Dating Method Characteristics Mineralisation References
Silurian (435-410 Ma)
LA-ICP-MS U-Pb Zircon
Intermediate/felsic volcanics
(Harris et al., 2004)
Devonian (400-350 Ma)
Rb/Sr Intermediate/felsic volcanics
Pb-Zn and Ag Carbonate hosted
(Harris et al., 2004)
Carboniferous (350 Ma)
LA-ICP-MS U-Pb Zircon, Rb/Sr
Basalt, rhyolite, and andesite
Pb - Zn (Harris et al., 2004; Intasopa, 1993)
Permian (300-260 Ma)
LA-ICP-MS U-Pb Zircon
Intermediate/felsic volcanics with limestone horizons
Cu-Au porphyry, epithermal Au, Skarn style
(Harris et al., 2004)
Triassic (245-210 Ma)
40Ar/39Ar, LA-ICP-MS U-Pb Zircon, SHRIMP
Intermediate/felsic volcanics, large granitoids
Fe, Cu, Au porphyry, epithermal, and skarn
(Harris et al., 2004; Intasopa, 1993; Rak, 1999)
Geochronology of Wang Yai
As part of this study and collaboration with ARC Linkage Project ‘Geochronology,
metallogenesis and deposit styles of the Loei Foldbelt in Thailand and Laos PDR’, five
rocks from the Wang Yai tenement were dated using LA-ICP-MS U-Pb zircon. Dating
was undertaken to confirm the Permian – Triassic age (Khin Zaw et al., 1999) of rocks at
Wang Yai. Of the five samples dated, only two samples; rhyolite and quartz phyric
Chapter 2 Regional Geology
- 17 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
breccia/sandstone contained zircons. The rhyolite (ER017162) was found during field
mapping outcropping in a dam wall at the Gift prospect and the quartz phyric
breccia/sandstone outcrops on Central Ridge and on the hill east of Conical Hill.
LA-ICP-MS U-Pb zircon dating of the quartz phyric rhyolite yielded an age of 321 ± 5
Ma (Late Carboniferous; Figure 2.4). Previous to this study, Carboniferous rocks in the
region were thought to consist of tholeiitic basalts and sedimentary packages such as
chert, shale, sandstone, conglomerate, and limestone (Harris et al., 2004). The age of this
rhyolite confirms that felsic magmatism occurred in the Loei Foldbelt. At Wang Yai
Permian – Triassic host volcanic rocks overlie the rhyolite.
LA-ICP-MS U-Pb zircon dating of the quartz phyric sandstone/breccia host rock yielded
an age of 247 ± 4 Ma (Lower Permian – Upper Triassic; Figure 2.5). This date is slightly
younger than previous 40Ar/39Ar dating of andesitic sandstone which gave an Early
300
320
340
360
380
400
0.04
0.06
0.08
0.10
0.12
15 17 19 21238 U/206 Pb
207Pb206Pb
data-point error crosses are 1 s
common Pb
ER17162 206Pb/238U age (207Pb cor.)Mean = 321±5 [1.4%] 95% conf.Wtd by data-pt errs only, 0 of 12 rej.MSWD = 0.69, probability = 0.75
Figure 2.4 LA-ICP-MS U-Pb Concordia plot– Quartz phyric rhyolite, Wang Yai, central Thaiand.
Chapter 2 Regional Geology
- 18 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Permian age of 293.8 ± 0.6 Ma (Khin Zaw et al., 1999). The 247 ± 4 Ma age for the
sandstone is however, consistent with LA-ICP-MS U-Pb zircon ages obtained for the host
volcanic stratigraphy (250 ± 6 Ma) at the Chatree gold mine (Meffre et al., 2005).
2.5 Age of hydrothermal alteration
Constraining the age of hydrothermal alteration at Wang Yai has important implications
in terms of determining its relationship with the nearby Chatree deposit and potential for
further exploration in the district. The hydrothermal alteration at Chatree deposit has
been bracketed between 256 Ma and 232 Ma (pers.comm., Harris 2005). This age range
is based on previous laser ablation 40Ar/39Ar dating of fine-grained adularia (as part of the
AMIRA project P390A) and LA-ICP-MS U-Pb zircon age of 238 ± 6 Ma for crosscutting
dykes (Meffre et al., 2005).
Figure 2.5 LA-ICP-MS U-Pb Concordia plot– Quartz phyric sandstone/breccia, Wang Yai, central Thailand.
200
600
1000
1400
1800
2200
0.04
0.06
0.08
0.10
0.12
0.14
0 10 20 30 40238U/206Pb
207Pb206Pb
data-point error crosses are 1 s
ER17646 Mean = 247±4 [1.6%] 95% conf.Wtd by data-pt errs only, 0 of 6 rej.MSWD = 0.40, probability = 0.85
Chapter 2 Regional Geology
- 19 –
Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand
Hydrothermal alteration assemblages at Wang Yai have been previously dated. Khin
Zaw et al. (1999) undertook laser ablation 40Ar/39Ar dating of fine-grained adularia of
intensely altered host rock at Wang Yai. The samples yielded ages of; 206.1 ± 0.3 Ma,
228.5 ± 0.2 Ma, 234.5 ± 0.3 Ma (Khin Zaw et al., 1999). As part of this project a further
2 whole rock samples, comprising of near total rock replacement of fine grained adularia,
were submitted for K-Ar dating (University of Queensland). The samples yielded ages of
123.8 ± 5.3 Ma and 111.4 ± Ma. This younger age is inferred to be due to thermal
resetting. K-feldspar is particularly susceptible to resetting by overprinting thermal
events such as deformation (faulting and metamorphism) and/or magmatism and
associated fluid flow.
Chapter 4 Gold-bearing quartz veins
47
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
4.1 Introduction
At Wang Yai, gold is hosted in a series of outcropping quartz-chalcedony vein systems
trending roughly north south. These quartz veins are best exposed as a line of
discontinuous, bulbous hills. In the southern part of the tenement, Erawan Mining has
individually named vein systems; these prospects include Conical Hill, Central Ridge, T1
Hill, and T4 Hill (Figure 4.1). Preliminary exploration in this area has included 5
diamond drill holes along a 190m long east-west transect at T1 Hill and two additional
holes on the north and south side of the transect. These holes have intercepted sporadic
stringer and stockwork quartz veins with minor gold mineralization. Gold is typically
hosted in comb quartz-carbonate-chlorite-illite ± adularia (hematite) veins. The northern
part of the tenement is characterized by vein systems collectively referred to as Gift. A
total of six vein arrays occur in the Gift portion of Wang Yai; of these five follow a
north-south trend, and one trends to the northeast. Exploratory trenches have been dug
across the veins, but as yet no diamond drilling has taken place.
This chapter describes the location and size of quartz veins and mineralised zones at
Wang Yai. Vein textures will be classified using the classification scheme of epithermal
quartz textures developed by Dong et al. (1995). A description of surface alteration
assemblages will be given for host rocks proximal to vein systems and a more detailed
description of primary hydrothermal alteration from T1 Hill drill core. Where present,
vein paragenesis and ore mineralogy will also be described. Discussions presented here
focus on the mineralogical and textural zoning in Wang Yai.
Chapter 4 Gold-bearing quartz veins
48 Figure 4.1 Aerial photograph of the Wang Yai tenement showing the locations and names of vein system, Central Thailand.
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
49
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.2 Conical Hill
Location: Conical Hill occurs at the southern end of the Wang Yai tenement. The hill on
which the centre vein outcrop occurs rises approximately 140m. Vein exposures include
massive crystalline quartz veins, quartz-chalcedony stockwork and sporadic host rock
outcrops.
The Conical Hill area comprises of two vein systems (see Figure 4.2). The main system
is a single in situ quartz vein (with anomalous Ag-Au values) outcropping on the western
slope of the hill (GR-1809600mN, 688615mE). Massive quartz-chalcedony outcrop
covers an area of 100m in strike (350˚N) and approximately 35m wide. The vein zone
narrows to several metres in the north and terminates at (GR1809625mN, 688615mE).
Large boulders (up to 1m) of quartz-chalcedony float are scattered on the slopes flanking
this outcrop and give the false impression of a considerably wider vein. Vein float
surrounding the main vein zone suggests that the vein zone pinches northward
(terminates at GR1809725mN, 6885785mE), while to the south the vein appears to swell
to a maximum width of 75metres (terminates at GR1809475mN, 688600mE). In part, the
quartz vein float includes host rock sandstone and breccia that is cut by quartz-
chalcedony stockwork. Based on the vein float distribution, it appears that the stockwork
zone covers an area approximately 100m wide and 325m long, and occurs immediately to
the east of the main massive quartz-chalcedony vein zone. Associated with this
stockwork zone is silicified, hematite-stained volcanogenic sandstone and breccia.
Two isolated outcrops of massive quartz and chalcedony also occur at Conical Hill.
These are considered here as off shoots of the main vein, described above. The first vein
outcrops in the northern and southern parts of a small dam wall (grid reference
1809475mN, 688615mE), approximately 20m south of the main vein zone. This vein
exhibits a massive sugary quartz texture, is 1 to 2m wide, strikes ~170˚ and dips 85˚E. It
was not found outcropping to the north or south of this location. The second vein was
Chapter 4 Gold-bearing quartz veins
found during excavation of a drill pad (GR1809620mN, 688645mE) and this vein is
approximately 12m long, 70cm wide and trends ~345˚/85˚E.
50 Figure 4.2 Fact map of Conical Hill showing the location of subcrop for main high grade vein system and the location of vein float for the second system to the west.
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
51
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.3 Quartz vein textures
Based on paragenetic and textural relationships, quartz veins in the main vein zone at
Conical Hill have been grouped into four distinct categories;
A-type veins are characterized by milky white, crustiform and colloform quartz
and chalcedony. They are typically associated with ‘ginguro’ (very fine
disseminated sulfide) ore. Where seen this ginguro ore is associated with the
micro-crystalline quartz bands. Thin adularia needles (now pseudomorphed by
quartz) occur on the margins of chalcedony bands. These ‘A-type’ veins also
display bladed quartz, silicified pseudomorphs of bladed calcite, and sericite
alteration. Well-developed pseudo-acicular textures after calcite (plus adularia)
also occur (see figure 4.3C).
B-type veins are characterized by brecciated vein with crystalline quartz cement
and clasts of chalcedony, limestone and sandstone. In the quartz cement moss
and ghost sphere textures usually occur. Typically, B-type veins do not host any
mineralisation.
C-type veins are characterized by massive sugary or saccharoidal quartz with
occasional moss and ghost sphere textures. C-type veins, like A-type at Conical
Hill are host to ginguro ore and disseminated pyrite (Figure 4.3A).
D-type veins cut all vein stages and are characterized by banded chalcedony and
minor clear often mineralised crystalline quartz which displays well developed
moss textures (Figure 4.3E).
4.3.1 Conical Hill (West)
Location: This vein is located approximately 175 metres immediately west of the main
Conical Hill vein. Although the vein does not outcrop, a line of scattered vein float
implies its occurrence. Mapping was made difficult by vegetation cover plus disruption
Chapter 4 Gold-bearing quartz veins
52
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
of the vein float by farming equipment. The distribution of vein float suggests a north-
south trend and a vein wide comparable to other vein systems in the area. The trend of
surface vein float is north south and has a strike length of approximately 475metres.
Float first occurs in the north at GR1809950mN, 688404mE and extends to
GR1809475mN, 688400mE in the south. The width of float can be up to 50m wide in
parts but is more commonly 25-30m. The vein is hosted in fine-grained clay + silica
altered volcaniclastic sandstone and breccia. Due to poor outcrop and access there is
potential for the vein to be more laterally extensive than what is interpreted here.
Quartz vein textures
The best gold grades are found in massive and banded sugary quartz and chalcedony
veins with minor comb quartz. Relict pits containing goethite, jarosite, and limonite after
sulphides occur. The highest gold assays (8.96g/t Au, 6.26g/t Au) occur in weathered
volcaniclastic host rock with recrystallised chalcedony 5-7cm wide stockwork. These
high assays may be a result of supergene enrichment rather than true gold grade, but
grades up to 4.38g/t Au have been found in relatively fresh, massive recrystallised
chalcedony.
Massive recrystallised chalcedony vein float exhibits cavities infilled with euhedral
growth zoned faceted quartz grains. Chalcedonic zones display well-developed moss
textures. Samples with these textures (including ER017417 and ER017522) have
representative assays of 1.27g/t Au and 4.38g/t Au, respectively. Zones of clear
translucent chalcedony tend to be preferred hosts of disseminated pyrite whereas white
milky chalcedony is barren.
Crustiform colloform vein float consists of an outer milky white chalcedony layer with
disseminated sulphide bands followed by alternating thin <4mm euhedral comb quartz
and 8mm chalcedony bands. The vein material in the centre has been completely
weathered but may have been late calcite infill.
Chapter 4 Gold-bearing quartz veins
B
53
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
A
C
A
Crystalline quartz
Ginguro banded ore Crystalline quartz
Quartz pseudomorphs of rhombic adularia
1 cm
D
Pseudo-acicular quartz
Crystalline quartz
F E
1 cm
Figure 4.3 A) - Massive crystalline quartz showing banded and disseminated ginguro ore (ER017427). B) -Photomicrograph of crystalline quartz in xpl (ER017427). C) - Pseudo-acicular texture with thin preserved quartz vein (ER017960). D) - Silica pseudomorphs of former rhombic adularia (ER017960). E) – Pink chalcedony crustiform colloform vein cutting grey crystalline quartz vein with disseminated and banded ginguro ore (ER017422). F) – Photomicrograph of crustiform colloform vein with moss textures and euhedral quartz(ER017960).
Grey microcrystalline quartz with disseminated ginguro ore
Chalcedony crustiform colloform vein
Chalcedony crustiform colloform vein
Well developed euhedral quartz crystals
Chapter 4 Gold-bearing quartz veins
54
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.3.2 Ore Mineralogy
Three sulphides have been identified in vein samples hosting ginguro ore at Conical Hill;
they include electrum, argentite, and pyrite. Argentite is the most abundant and
constitutes 90% of all sulphides. The sulphides are largely hypogene in origin.
Electrum, argentite and pyrite occur in dark grey to black ‘ginguro’ bands. These
metallic ginguro bands are typically associated with crystalline quartz, flamboydial
quartz, lattice bladed quartz, and tabular adularia pseudomorphs. Supergene electrum also
occurs as inclusions in hematite and goethite lined cracks.
Argentite: Argentite (AgS) occurs as small less than 1mm aggregates that most
commonly occurs in thin >0.05mm bands, or on the edges of quartz grains (Figure 4.4A).
Banded aggregates occur in the hinges of colloform chalcedony and in association with
lattice bladed quartz pseudomorphs the resemble lattice bladed calcite. Argentite is
intimately associated with electrum grains and where seen, it commonly occurs
immediately adjacent to electrum. The presence of argentite is clearly visible in hand
specimen and occupies up to 95% ginguro ore mineralogy. Microprobe analysis (see
Table 4.1) detected trace amounts of Se (0.2 – 2.0 wt.%) and Te (0.1 wt.%). Anomalous
Cu was detected in one grain (14 wt.%), whereas all other argentite grains have Cu values
below the detection limit.
Electrum: In high grade samples (101g/t Au, 63g/t Au) individual electrum grains are
visible in ginguro bands. In thin section they occur as large (>0.5 mm) free grains on the
margins of quartz or within quartz. Electrum and argentite preferentially occur in fine-
grained quartz horizons, colloform chalcedony and lattice bladed quartz. Electrum also
occurs as smaller (<5 mm) blebs within, or on the margins of argentite grains. Electrum
has also been seen in small cracks within argentite grains. Less commonly small
electrum blebs are found as free grains in or along the margins of single quartz crystals.
Supergene electrum also occurs in weathered samples cut by cracks filled with hematite
and goethite. Electrum occurs as small rounded intergrowths within hematite bands.
Chapter 4 Gold-bearing quartz veins
Relict sulphides display colloform weathering textures after hematite replacement. It is
these secondary colloform hematite grains that commonly host small blebs of electrum.
To determine the composition of electrum, microprobe analysis was conducted on nine
different electrum grains from three different vein samples (see Table 5.1) Microprobe
analysis along transects of single electrum grains has revealed a gradation from a silver
rich rim to a gold rich core. This was reflected in the large variation (from 28 to 61%) of
Ag in electrum. However, this large variation may reflect errors in positioning the
electron beam rather than a true gradation of silver to gold. Trace amounts of Cu (4.2
wt%) were detected in one grain which also contained detectable S (0.1 wt%), Se
(0.1wt%), and Te (0.1 wt%). Tellurium was present as trace amounts (0.1 wt%) in all
grains, whereas S and Se were restricted to grains with appreciable Cu. Gold fineness
values for the electrum grains were calculated using the equation (Au/Au+Ag×1000).
Values ranged from 610 to 490 and are consistent with values from other low
sulphidation deposits such as Chatree (see Figure 5.3; Dedenczuk, 1998; Kromkhun,
2005 ).
Table 4.1 Summary of microprobe analyses of argentite at Wang Yai, central Thailand.
Sample no. Au (wt%) Ag (wt%) Cu (wt%) S (wt%) Se (wt%) Te (wt%) Total
(wt%) ER017957 -0.1 83.3 0.0 14.0 1.9 0.1 99.4 ER017957 0.0 82.8 0.0 11.6 0.7 0.1 95.4 ER017426 1.1 70.1 14.5 13.2 0.0 0.1 99.1 ER017426 -0.1 84.3 0.0 17.0 0.7 0.1 102.2 ER017426 0.2 82.4 0.0 11.8 3.3 0.1 98.1 ER017426 -0.2 83.9 0.0 13.3 2.0 0.1 99.4
55
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
56
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
1 cm
Ginguro bands Ginguro band
Electrum
Argentite
Electrum
Electrum
Argentite Pyrite
Argentite
Colloform growth zoned replacement of argentite
Argentite
Electrum Electrum
Figure 4.4. A) – Ginguro banded ore in crystalline quartz vein (ER017957). B) Photomicrograph in reflected light showing a electrum grain associated with argentite dominated ginguro ore (ER017690). C) – Photomicrograph of an angular electrum grain joined to argentite (ER017690). D – Photomicrograph of electrum occurring within cracks of argentite (ER017957). E) – Photomicrograph of electrum occurring as small inclusions within large argentite grain (ER017957). F) – Supergene replacement of argentite by hematite forming radial colloform growths (ER017960).
Chapter 4 Gold-bearing quartz veins
Table 4.2 Summary of microprobe analyses of electrum at Conical Hill, Wang Yai, central Thailand
Mineral Sample no. Au (wt%) Ag (wt%) Cu (wt%) S (wt%) Se (wt%) Te (wt%) Total (wt%) ER017957 60.8 40.0 0.0 0.0 0.0 0.1 101.2 ER017957 40.6 42.1 4.2 0.1 0.1 0.1 89.2 ER017960 61.5 40.3 0.0 0.0 0.0 0.1 102.1 ER017960 53.6 46.5 0.0 0.0 0.0 0.1 100.4 ER017426 33.5 31.1 0.0 0.0 0.0 0.1 64.8 ER017426 59.5 40.6 0.0 0.0 0.0 0.0 100.4 ER017426 59.4 41.8 0.0 0.0 0.0 0.1 101.5 ER017426 45.0 42.0 0.0 0.0 0.0 0.1 87.3 ER017426 59.5 40.6 0.0 0.0 0.0 0.0 100.4
Conical Hill gold fineness frequency histogram
0
1
2
3
4
5
400 450 500 550 600 650 700 750Gold fineness
Freq
uenc
y
Figure 4.5 Frequency histogram of gold fineness values for Conical Hill, Wang Yai, central Thailand.
57
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
Chalcopyrite and Pyrite: Chalcopyrite and pyrite are very minor stages and their
occurrence is relatively rare. One chalcopyrite grain was observed on the edge of a
quartz grain in sample ER017426. Cubic pyrite grains are relatively more common but
have largely weathered to hematite, so the correct identification as pyrite is difficult
except for the distinct rhombic shape.
4.4 Central Ridge
Location: Central Ridge is a prominent hill up to 155 m high and is found approximately
750m north of Conical Hill and 500m south of T1 Hill. Vein exposures include massive
opaline silica to chalcedony, chalcedony quartz crustiform colloform and sporadic host
rock outcrops.
The Central Ridge vein system outcrops at ‘monument’ (GR1809750mN,688825mE) and
continues as subcrop and float train along a ~170 strike for approximately 580m and
eventually terminates at the pinnacle of Central Ridge (GR1810275mN, 688700mE;
Figure 4.7). The vein pinches at the northern and southern ends and swells in the middle
section, but this observed relationship may be due to float in the steeper middle sections
moving down slope. The inferred maximum vein width (up to 50m wide) occurs
approximately 250m from ‘monument’ (GR1810025mN, 688750mE). Here the vein
system is hosted in volcanogenic breccia and sandstone and grades upslope to a silicified
mudstone, limestone, and a quartz phyric breccia at the pinnacle.
58 Figure 4.6 Looking south from T1 Hill towards Central Ridge, Wang Yai, central Thailand (photo pers.comm Stuart Smith)
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
59 Figure 4.7 Fact map of Central Ridge showing distribution of vein float and subcrop, Wang Yai, central Thailand.
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
60
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.4.1 Quartz vein textures
Along the entire strike of the Central Ridge vein, gold assays are low and most samples
have gold values below the detection limit. The only significant gold grade (0.68g/tAu,
0.36gt/Au, 0.26gt/Au) occurs in the northern most 200m of the vein (Figure 4.7). South
of this area, vein quartz are all below detection limit values. Quartz textures along strike
of this vein display a zonation which correlates with increase in gold grade towards the
northern end. For example opaline silica and chalcedony (<0.01g/t Au) dominate in the
south of the system, whereas in the north, chalcedony, crustiform colloform veins
(0.36gt/Au, 0.26gt/Au) dominate. The boundaries between quartz textures are
gradational and textures belonging to both zones occur in transition. At Central Ridge
the veins have been subdivided into three types; A and B-type quartz veins occur at the
southern end of the ‘monument’ vein, whereas C-type quartz is found in the north.
A-type veins are characterized by well-developed crustiform banding of
alternating chlorite (1-3 mm) and chalcedony/opal bands (2-5 mm). Cockade and
comb quartz textures are marginal to banded chalcedony and opal (Figure 4.9 A
and B). Chlorite occurs as bands (with hematite dusting) or sphere like shapes
closely associated with chalcedony. Approximately 200 m north of the
‘monument’ vein, A-type veins are much thinner (1 cm) and occur as isolated
vein adjacent to the main vein zone (effectively as a vein selvage).
B-type veins are predominant in the southern end of this vein and are interpreted
to be a late stage chalcedony flooding event (Figure 4.9 C). They cut A-type
veins at high angles and consist of massive dark yellow-brown chalcedony 20-30
cm wide veins which contain occasional lithic clasts and zones of hydrothermal
breccia. Cavities within the chalcedony are in-filled with euhedral growth zoned
faceted quartz grains. Chalcedonic zones display well developed moss textures
and are unmineralised.
Chapter 4 Gold-bearing quartz veins
C-type The inside margin of A-type veins have C-type chalcedony veins with
well developed crustiform colloform banding and cockade and comb quartz
textures (Figure 4.9 D, E and F). The margins of comb quartz bands are host to
pyrite mineralization (Figure 4.8 A). Pyrite mineralisation is interpreted to be the
host of gold mineralisation although electrum in not visible in hand specimen.
Assays for C-type vein samples include 0.68gt/Au, 0.36gt/Au, and 0.13gt/Au. C-
type veins are followed by breccia stage (D) with chalcedony cement and clasts of
crustiform colloform banding.
4.4.2 Ore Mineralogy
Only two samples from Central Ridge display sulphides. These include ER016933
(0.68gt/Au) and ER016955 (0.26gt/Au). In both samples the sulphides have been
replaced by colloform secondary hematite. The euhedral shape of sulphides is still
preserved and based on this together with microprobe data they are interpreted to be
chalcopyrite and pyrite. Chalcopyrite and pyrite occur in the fine grained crystalline
quartz horizons of crustiform colloform chalcedony (Figure 4.8, A). Electrum was also
thought to occur but microprobe analysis confirmed these grains as unaltered
chalcopyrite. Relatively pristine grains of chalcopyrite forming on the margins of quartz
crystals were observed in sample ER016955 (Figure 4.8, B).
61
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
A B
B Chalcopyrite grain
A
Chalcedony
Euhedral quartz
Crystalline quartz
Weathered sulphide bands
Figure 4.8 A) Photomicrograph of a cross section through a typical chalcedony quartz crustiform colloform band. Weathered sulphide bands occur within the crystalline quartz bands (sample ER016933). B) Photomicrograph ofa rare isolated chalcopyrite grain occurring in a quartz grain (sample ER016955).
Chapter 4 Gold-bearing quartz veins
62
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 4.9 A) – Opaline silica chlorite vein at the southern end of the Central Ridge vein system (sample ER017246). B) – Photomicrograph of opaline silica vein (sample ER017246). C) - Late stage yellow chalcedony flooding, cross cutting opaline silica chlorite vein (sample ER017249). D) – Photomicrograph of thin alternating, crustiform colloform bands with quartz and chalcedony. Quartz displays flamboyant textures. E) – Chalcedony crustiform colloform bands (sample ER017422). F) – Photomicrograph of alternating chalcedony and quartz crustiform bands. Euhedral quartz crystals infill open spaces (sample ER016955).
B A
Opaline silica
Opaline silica Chlorite
1 cm
D C Late stage chalcedony flooding
Flamboyant quartz
Fine-grained quartz Opaline silica + chlorite 1 cm
E F Quartz crystal filling open spaces
Chalcedony bands Crustiform colloform banded vein
1 cm
Chapter 4 Gold-bearing quartz veins
4.5 T1 Hill
The T1 Hill prospect is located approximately 425 immediately north from the top of
Central Ridge. T1 Hill is the only prospect in the Wang Yai tenement that has been
drilled (three diamond drill holes by Phelps Dodge Ltd. and an additional four holes
drilled by Thai Global Ventures Co., Ltd.). Vein exposures include massive crystalline
quartz veins, quartz-chalcedony stockwork and sporadic host rock outcrops.
At the top of T1 Hill quartz vein subcrop starts at (GR1810675mN, 688750mE), trends
north, and curves around to the northwest terminating at (GR1810800mN, 688650mE).
The strike length of vein float from south to the northwestern end is approximately 150m
and has a width of 25 metres (Figure 4.11). Vein dimensions are inferred from the
distribution of vein float so the actual vein size is probably much smaller. Stockwork
veining in in situ volcaniclastic breccia and sandstone host rock occurs on the northern,
eastern, southern, and western sides of the vein.
In addition to the descriptions of surface quartz textures and ore mineralogy, more
detailed descriptions of the vein textures, alteration assemblages, and ore mineralogy will
be presented for T1 Hill. Observations presented here focuses on a single drill hole (i.e.,
WYRD016), which was given the most attention during the logging exercise.
63 Figure 4.10 Looking north from Central Ridge towards T1 Hill, Wang Yai, central Thailand.
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
64
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 4.11 Map of T1 Hill showing location of the main vein system and associated quartz veintextures, Wang Yai, central Thailand
Chapter 4 Gold-bearing quartz veins
65
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.5.1 Quartz vein textures
Four types of quartz veins occur on the surface of T1 Hill.
A-type veins are characterized by 2-3cm milky white chalcedony bands and
darker coarse grained quartz exhibiting pseudoacicular textures after calcite. Thin
bands of sulphides occur in between the chalcedony and quartz bands. The
sulphides seem to be associated with quartz bands rather than chalcedony (Figure
4.12, A).
B- type veins consist of an breccia stage that comprises of angular lithic silicified
sand and siltstone clasts in a clear dark grey crystalline quartz matrix. Angular
fragments of unmineralised chalcedonic, crustiform colloform vein also occur
(Figure 4.12, B).
The grey crystalline quartz matrix displays ghost sphere textures, zoned quartz,
lattice bladed quartz and disseminated sulphides. Lattice bladed quartz after
calcite seems to be associated with fine grained ore baring bands.
Some samples exhibit weathered hematite bands which may have been ginguro
ore. Sulphides are generally restricted to fine quartz and lattice bladed quartz
zones. Most B-type veins exhibit gold assays >1.0 g/t Au but these high values
may be attributed to supergene enrichment rather than true values as most of the
samples are intensely weathered.
C-type veins consist of massive milky white recrystallised chalcedony with
abundant cockade structures that have centres composed of euhedral quartz
grains. Moss textures are also present. No mineralisation is visible (Figure 4.12,
C).
D-type veins cross cut all other vein stages and consist of 1-2cm comb quartz
with euhedral growth zoned.
Chapter 4 Gold-bearing quartz veins
66
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
D
F
Pseudo-acicular quartz
1 cm
A
Ginguro bands
B
Lithic clasts
Crystalline quartz Crystalline quartz matrix 1 cm
D C
Cockade quartz
1 cm
E
Adularia selvage
Quartz vein
Adularia Bladed calcite
Pyrite
Late calcite vein Comb quartz vein
F
Figure 4.12 A) - Grey pseudo-acicular quartz band with sulphide and white crystalline quartz selvages(ER016901). B) – Hydrothermal breccia vein with silicified lithic clasts. Matrix is comprised of crystalline quartz and disseminated sulphides (ER016907). C) – Massive aggregates of comb and crystalline quartz (ER016902). D) – Photomicrograph of lattice bladed calcite band with rhombic adularia and rhombic pyrite(WYRD016@243.7 metres). E). – Photomicrograph of adularia occurring on a comb quartz vein selvage(WYRD016@243.7 metres). F) – Photomicrograph of late stage unmineralised calcite vein cutting comb quartzcarbonate vein (WYRD016@243.7 metres).
Chapter 4 Gold-bearing quartz veins
67
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.5.2 Detailed vein and alteration assemblage
Four different alteration assemblages and associated vein types have been observed
during core logging at T1 Hill. They include silicic, K-feldspar, chloritic, argillic and
phyllic alteration assemblages.
4.5.2.1 Silicic alteration assemblages
The earliest alteration stage consists of pervasive silica flooding of andesitic host rock
and rare chalcedony horizons (up to 2 m thick). These pervasive alteration zones are
associated with, and cut by quartz veinlets (4 to 30 mm) . Chalcedony horizons are dark
brown/ grey chalcedony with chlorite hematite, pyrite selvages and singular anhedral
quartz crystals.
The second stage of silicic alteration assemblages is variable in both mineralogy and
textures. Veins in this stage do not contain all the phases listed below but generally have
at least two of the characteristics. This stage is characterized by dark/light grey 1-4 cm
comb quartz with chlorite, hematite, calcite and 1-2 cm sericite haloes (Figure 4.12, F).
Calcite usually occurs as infill in the thicker veins >4 cm whereas the smaller veins
calcite occurs along the vein selvage. Fine grained crystalline quartz, chlorite and
hematite form bands along vein selvages. Crystalline zones of chalcedony occur in
conjunction with euhedral quartz and often form on the upper margins of euhedral quartz
grains. Some phases exhibit thick rhombic adularia selvages and zones of very well
developed lattice bladed silica pseudomorphs after calcite (Figure 4.12, D and E). Lattice
bladed textures tend to preferentially occur with fine grained saccharoidal quartz and
adularia rhombs. Pyrite content is mostly confined to the banded ‘smoky green’
chlorite/quartz rich phases. Wall rock located pyrite also increases around the presence
of smoky green vein phases.
Fine grained quartz crystals that flood wall rock adjacent to veins infill vesicles after K-
feldspar and pervasively alter the host rock groundmass. Grain size is variable with
Chapter 4 Gold-bearing quartz veins
68
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
larger growth zoned quartz occurring in amygdales and finer grained quartz occurring in
thin veinlets and the host rock groundmass.
The textures and abundance of chalcedony and quartz are significantly different across
the vertical interval of T1 Hill. Quartz veins from the surface are dominated by
crystalline quartz and chalcedony and minor comb quartz. In contrast, drill core veins are
dominated by comb quartz with well developed growth zoned euhedral crystals and very
little chalcedony. According to Morrison et al., (1991) quartz textures can be used to
determine the approximate level in an epithermal system and this will be discussed
further in the interpretation section.
4.5.2.2 K-feldspar alteration assemblages
K-feldspar alteration assemblages are wide spread at T1 Hill and are closely associated
with pervasive silicic alteration. This alteration zone is characterised by low temperature
K-feldspar, adularia.
K-feldspar alteration occurs as thin pink bands (>1 mm) within quartz carbonate veins
but more commonly along vein selvages as rhombic adularia (>0.5 mm) aggregates.
Bands of adularia within quartz carbonate veins are generally associated with silica
pseudomorphs of lattice bladed calcite. K-feldspar alteration also occurs as thin selvages
around amygdales in andesitic host rock and also replaces large euhedral plagioclase
phenocrysts and small plagioclase laths in the groundmass.
K-feldspar staining was undertaken at the University of Tasmania using hydrofluoric acid
on 20 samples from drill hole WYRD016. K-feldspar staining was conducted to
determine the distribution of K-feldspar alteration assemblages relative to the gold-
bearing quartz veins. Samples used for chemical staining were selected at intervals of 5m
from 135 m to 248 m down hole. Samples included those immediately adjacent to the
veins and altered host rock upto 50 metres away from the vein. Results from staining
Chapter 4 Gold-bearing quartz veins
showed a clear increase in K-feldspar alteration down hole towards the quartz-chlorite-
carbonate vein zones (Figure 4.13).
69
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
1 cm
Figure 4.13 K-feldspar stained andesitic host rock (sample WYRD016@280 metres).
4.5.2.3 Argillic and phyllic alteration assemblages
At T1 Hill clay alteration haloes around veins have been identified in hand specimen and
also under the microscope. In hand specimen sericite alteration forms conspicuous pale
yellow/green 4-8 cm haloes around quartz carbonate veins. In thin section clays
selectively replace adularia, plagioclase and pyroxene phenocrysts, and vein calcite.
Sericite also occasionally pervasively replaces the groundmass in andesitic clasts. In
order to identify the type of phyllosilicate assemblages PIMA analysis was applied to
zones exhibiting argillic alteration. However, the analysis failed to identify the presence
of any clays although they are clearly visible in hand specimen and thin section. This
may have been due to the host rock being intensely silicified and containing abundant
disseminated pyrite.
4.5.2.4 Chloritic alteration assemblages
Pervasive and selective chloritic alteration assemblages are common at T1 Hill. Selective
alteration of glass, plagioclase, pyroxene, and amygdales occurs in coherent andesite
Chapter 4 Gold-bearing quartz veins
70
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
facies, sandstone facies, and breccia facies. Pervasive chloritic alteration is best
developed around the hydrothermal zone where chlorite replaces the entire groundmass
and phenocryst assemblages. Chlorite also occurs along vein selvages in conjunction
with hematite, epidote and sericite and is not necessary associated with hydrothermal
zones.
4.5.2.5 Propylitic alteration assemblages
In epithermal systems propylitic alteration is considered by Buchanan (1981) to be a wide
spread assemblage commonly forming halos of hundreds of metres around vein zones
and is usually considered to be post-ore. Similarly at T1 Hill it has been interpreted from
overprinting relationships of K-feldspar, and phyllic assemblage that propylitic alteration
occurred later or post ore.
Propylitic alteration assemblages are well developed away from the hydrothermal zone.
They include the following assemblages; chlorite, epidote, sericite, quartz, albite,
carbonate, pyrite and hematite. PIMA analysis conducted on drill core distal from the
hydrothermal zone shows that chlorite, epidote and phrenite are the dominant
phyllosilicates. Chlorite and epidote selectively replace K-feldspar, hornblende and
plagioclase andesite phenocrysts. Where feldspars are not replaced by chlorite and
epidote they exhibit albite replacement with a sericite dusting. Epidote occurs along vein
selvages in association with sericite, chlorite and pyrite. Hematite and silica pervasively
alter the groundmass in all lithofacies. Pyrite is usually associated with these
assemblages.
4.5.3 Ore Mineralogy
In surface vein float samples ore is hosted in grey/back disseminated sulphide bands.
Pyrite, chalcopyrite, and minor electrum are the dominant sulphides. Pyrite grains
assume subhedral to euhedral shapes with an average size of 50 µm. 15 % pyrite grains
exhibit sizes up to 350 µm. Pyrite typically occurs in the vein matrix and within clasts of
Chapter 4 Gold-bearing quartz veins
the breccia. Crustiform colloform banded veins do not exhibit any sulphides. Most of
the pyrites are weathered especially within the hematite altered zones. In these zones it
loses its colour changing to a light blue to brown and exhibits growth zoned colloform
texture. Only one 70 µm chalcopyrite grain was observed and this was partially replaced
by secondary covellite. Sulphides are generally restricted to small bands along the
boundaries between saccharoidal and mosaic quartz although they also occur as
disseminations on the edges of quartz. Supergene sub-rounded electrum (25 µm) was
observed within a hematite weathered crack.
In drill core, pyrite and chalcopyrite are the dominant sulphides. Large euhedral 200-300
µm pyrite grains preferentially occur within the wallrock rather than quartz carbonate
veins. Occasional small grains <50 µm occur in the centre and along the selvages of
quartz carbonate veins. Chalcopyrite occurs as very minor <50 µm grains and is
restricted to quartz carbonate veins. Chalcopyrite can occur as free grains or as
inclusions within pyrite (Figure 4.14).
71
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
A B Pyrite
Pyrite
Chalcopyrite Chalcopyrite
Figure 4.14 A) Photomicrograph showing two pyrite grains occurring with two chalcopyrite grains (sample WYRD016@234 metres). B) Photomicrograph of a large pyrite grain with chalcopyrite grains occurring in cracks of pyrite (sample WYRD016@234 metres).
Chapter 4 Gold-bearing quartz veins
72
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.6 T4 Hill
Location: T4 Hill occurs at the northeast corner in the southern portion of the Wang Yai
tenement. The hill on which the vein system is located rises approximately 135 m. The
vein system trends northeast and occurs as scattered float flanking the base of the
southeast side of the Hill. Intensely weathered vein float starts at (GR1811050 mN,
689600 mE) and extends for 200 metres terminating at the edge of rice paddies
(GR1811175 mN, 680700 mE).
Quartz textures: Quartz textural analysis is limited for T4 Hill due to the sample set
consisting of 3 vein samples, the remainder are silicified wall rock. The highest assays to
date include 3.20 g/t Au, and 0.69 g/t and these values are likely to reflect supergene
enrichment rather than hypogene as they occur in brecciated and highly weathered
samples.
The vein samples are characterized tabular aggregates of coarse grained barite crystals
which infill vugs within micro crystalline quartz dominated groundmass. Very thin
quartz lattice bladed textures occur and have barite crystals growing perpendicular from
the blade direction.
4.7 Gift Prospect
The Gift prospect is located in the northern part of the tenement and comprises of 6
different vein systems. They will be referred to as A, B, C, D, E and F (Figure 4.15).
Location:
Vein system A: occurs in the northern end of the Gift prospect flanking the western side
of Song Hill (Figure 4.15). Vein float occurs in alluvium at the base of Song Hill and in
eucalyptus plantations on the flanks of Song Hill. The vein starts at (GR1812800 mN,
687525 mE) in the south and extends approximately 300m due north and terminates at
(GR1813025 mN, 687525 mE). The approximate width of vein float ranges from 25-
Chapter 4 Gold-bearing quartz veins
30m and is hosted in silica ± pyrite ± hematite ± clay altered volcanic breccia and
sandstone.
73 Figure 4.15 Location of vein systems, vein float and surface alteration in Giftprospect, Wang Yai, central Thailand.
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
74
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Vein system B: occurs as float on the eastern side of Song Hill along the same latitude as
vein system A (Figure 4.15). In the south, vein system B starts at (GR1812650 mN,
687975 mE) and extends approximately 500m immediately north terminating at
(GR1813150 mN, 687975 mE).
Vein system C starts at the northern end of B and trends north east towards the top of
Kham Hill. Vein float occurs along this trend for approximately 500m (Figure 4.15).
Vein system D occurs at the northeastern end of C and trends north south from the
southwestern base of Kham Hill (Figure 4.15). Vein float is conspicuous with an average
width of up to 50 metres. It is hosted in intensely silicified hematite volcanic breccia to
the west and epidote-chlorite massive volcanic conglomerate to the east (Figure 4.15)
Vein system E is located directly south of system B and extends to the southern end of
Suwan Hill. This vein system is characterized by very sparse scattered vein float. The
trend of the vein is difficult to determine as barite quartz veins in patchy outcrop strike
east west and the trend of float is north south.
Vein system F could potentially be a southern extension of ‘A’-system to the north but
will be considered here as a separate system.
4.7.1 Quartz textures:
Vein System A: This vein system exhibits the highest gold grade in the Gift prospect.
Quartz vein textures in system A are dominated by massive white crystalline quartz
(Figure 4.17 C). Massive white crystalline quartz veins consist of randomly oriented
quartz grains with variable sizes. Primary textures are the dominant quartz texture and
include massive, moss, cockade, comb quartz textures. Recrystallisation textures include
ghost bladed and saccharoidal quartz, tabular aggregates of former barite, calcite or
adularia also occur (Figures 4.17D, A, B). Former adularia rhombs now replaced by
silica form thin bands associated with lattice bladed and ghost bladed textures.
Chapter 4 Gold-bearing quartz veins
75
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Mineralisation preferentially occurs in zones where ghost bladed, tabular aggregates and
pseudomorphs of adularia occur.
Vein system B: Vein system B is exhibits the lowest gold grade in the Gift prospect.
Most samples are below detection limits with only two samples exhibiting grade >0.01
g/t Au. Three main vein stages occur in this system; chalcedony crustiform, massive
chalcedony quartz and a later breccia stage consisting of chalcedony vein clasts.
Chalcedony crustiform vein exhibits well-developed thin chalcedony bands with
occasional thin discontinuous sulphide bands. The vein centre consists of bladed
pseudomorphs after either calcite or barite. The late breccia stage consists of angular (5-
10 mm) brown-yellow chalcedonic vein fragments which display cockade and crustiform
colloform textures. The matrix is typically fine-grained quartz with small >1 mm vein
and wall rock fragments.
Vein system C and D
Similarly to system B, system C exhibits a very low gold grade with the highest assay
being 0.02 g/t Au. Quartz vein textures in this system also share similarities with B in
that they exhibit well-developed chalcedony dominated crustiform colloform banded
veins (Figure 4.17 E and F). Primary quartz textures include crustiform, colloform, moss,
comb and zonal quartz. Recrystallisation textures are less common and include minor
flamboyant and ghost sphere textures.
Vein system E
Vein system E is interpreted to be a southern extension of system B and similarly to B it
is characterized by very low gold grade. Only 4 samples from this system have been
collected so it is difficult to get a true representation of the dominant quartz textures.
There is one sample that exhibits well developed bands of former adularia and bladed
calcite. This sample (ER017494) has a gold grade of <0.01 g/t Au which contradicts
Chapter 4 Gold-bearing quartz veins
what has been observed in other vein systems in that gold grade is associated with
adularia and bladed calcite.
Vein system F
The main vein phase in this system consists of vuggy hydrothermal quartz breccia with
angular quartz chalcedony vein clasts and host rock clasts. Vein clasts range in size from
3 cm to 2 mm and commonly display pseudo-acicular recrystallisation textures. Matrix
quartz exhibits a white or grey texture and is composed of variable sized euhedral quartz
grains (1-2mm) which occasionally display zonal, cockade, flamboyant, moss textures
and fine-grained microcrystalline white quartz. In fresh samples pyrite is abundant
(25%) and occurs as euhedral disseminations throughout the vein (Figure 4.16A). It
preferentially occurs in the matrix rather than vein clasts. There is evidence to suggest
weathered samples also contained up to 25% disseminated pyrite as sulphide pits now
replaced by goethite and limonite are common. Quartz textures associated with gold
grade tend to include grey and white microcrystalline quartz (Figure 4.16, A and B). The
distribution of pyrite also seems to be associated with gold grade as samples with either
fresh disseminated pyrite or weathered sulphide pits yield the highest assays.
76
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
B A Grey crystalline quartz
Disseminated sulphides Massive crystalline quartz
1 cm
Figure 4.16 A) Hand specimen from ‘F’ system showing grey crystalline quartz with disseminated pyrite and chalcopyrite (sample ER017479). B) Photo micrograph from ‘F’ system showing randomly orientated and variablesized massive crystalline quartz (sample ER017479).
Chapter 4 Gold-bearing quartz veins
77
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Quartz crystals B A Ghost bladed quartz
Crystalline quartz 1 cm
D Moss quartz aggregates
C
Tabular aggregates
Moss quartz
4.17 A) Hand specimen from ‘A’ system showing crystalline milky quartz and well-developed euhedralaggregates of quartz (sample ER017470). B) Photomicrograph from ‘A’ system show ghost bladed quartz aftercalcite (sample ER017474). C) Photomicrograph showing typical quartz moss textures in ‘A’ zone (sampleER017474). D) Photomicrograph showing moss, and tabular aggretates or either barite, adularia, quartz orcalcite in ‘A’ zone (sample ER017474). E) Hand specimen sample of typical chalcedony and opaline silicacrustiform colloform banded veins in ‘C’ zone (sample ER017509). F) Photomicrograph of chalcedonycrustifrom colloform banded vein from ‘C’ zone (sample ER017509).
1 cm
E
Chalcedony crustiform colloform bands
F
Chalcedony crustiform colloform bands
Chapter 4 Gold-bearing quartz veins
78
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.7.2 Alteration
Surface alteration in Gift prospect (see Figure 4.15) comprises of four assemblages.
They include silica + hematite, silica + pyrite, hematite + clay, and epidote + chlorite.
The distribution of each assemblage is shown in Figure 4.15. Silica + hematite alteration
is the most extensive and is characterized by pervasive silicification and weathering of
mafic minerals to hematite. This alteration forms conspicuous red, silicified outcrops on
hill tops and is usually proximal to vein zones. The degree of silicification increases with
proximity to the vein and stockwork systems. Hematite + clay alteration usually forms
along the fringes of silica hematite alteration distal from intense silicification associated
with vein and stockwork zones. Silica + pyrite alteration occurs in association with the
high grade vein zones (A, E). It is characterized by intense pervasive alteration of host
rock forming very hard subcrop zones containing up to 25% disseminated pyrite.
Chlorite + epidote alteration is similar to a typical propylitic style assemblage and occurs
to the north east in volcanic conglomerate.
4.7.3 Ore Mineralogy
Sulphides observed in the Gift prospect were from high grade vein systems (i.e, A and E)
Low grade systems were completely barren of sulphides although evidence of hematite
weathering bands and sulphide pits indicates that veins may have been host to
mineralisation. Sulphides in vein systems A and E include pyrite, chalcopyrite, electrum
and secondary covellite. Pyrite is the most abundant sulphide and occurs in discrete
bands or disseminations associated with chalcopyrite. Pyrite grains range in size from 50
to 110 μm and occasionally contain inclusions of chalcopyrite. Chalcopyrite occurs (20
to 100 μm) as isolated grains on the edge of quartz crystals or with pyrite aggregates.
Covellite occasionally replaces the entire chalcopyrite grain but more commonly replaces
only the rims (Figure 4.18A). Electrum occurs very rarely and was observed once in thin
section (sample ER017474). The electrum is approximately10 to15 μm (Figure 4.18B)
and forms along the edges of intersecting quartz grains. Microprobe analysis of this grain
Chapter 4 Gold-bearing quartz veins
showed trace amounts of Fe (0.12wt %), Bi (0.12wt %). Tellurium was below detection
limits and gold, silver content was 72 wt % and 26 wt % respectively. Gold fineness
value for the electrum grain was 720.
B
79
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
A B Chalcopyrite
4.8 Mineralogical and textural zoning
One of the major aims of this project is to determine what level the surface vein systems
at Wang Yai are at with reference to current quartz textural, gangue mineralogy and ore
mineralogy zonation in current epithermal models. Constraining the approximate level of
formation can implicate whether there is potential for mineralisation at depth or the
system has been eroded away. In the above section, vein systems have been described in
terms of dominant primary, recrystallisation, and replacement quartz textures. Using the
descriptions of quartz vein textures this discussion will focus on the lateral zonation
between the different vein systems and then lead into interpretation of why and how
quartz textures and gold grade differ between each vein system.
Quartz textures
Figure 4.18 A). Covellite, pyrite and chalcopyrite, covellite is replacing the chalcopyrite rim (sample ER017479). B) Isolated electrum grain occurring on the edge of intersecting euhedral quartz crystals (sample ER017474).
Electrum Pyrite
Quartz Covellite
Chapter 4 Gold-bearing quartz veins
80
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
A summary of quartz textures for each vein system in Wang Yai is shown in Table 4.3.
At Conical Hill primary massive, moss, and crustiform textures predominate. Massive
and moss textures are likely to have formed from precipitation of a silica gel (Dong et al.,
1995). Recrystallisation mosaic textures are common and are a result of recrystallisation
of massive chalcedony or amorphous silica (Lovering, 1972). Replacement textures such
as saccharoidal, pseudo-acicular and lattice-bladed quartz are associated with ginguro
bands and are a likely product of recrystallisation from either calcite, adularia, or barite
phases (Dong et al., 1995).
Quartz vein textures at Central Ridge differ significantly from Conical Hill. At Central
Ridge, primary and recrystallisation textures in chalcedony vein phases predominate and
replacement textures are absent. Primary quartz textures such as crustiform colloform
banding are likely to have formed from episodic pressure release (Buchanan, 1981) and
precipitation from a silica gel precursor (Adams, 1920). Recrystallisation textures such
as mosaic, feathery, and ghost-sphere are indicative of recrystallisation of amorphous
silica or chalcedony (Dong et al., 1995).
At T1 Hill quartz vein textures share many similarities with Conical Hill in that they
exhibit well developed replacement textures such as saccharoidal, pseudo-acicular, ghost-
bladed and lattice-bladed quartz. The presence of these textures and their close
association with sulphides suggests that ore bearing phases precipitated adularia, calcite
and quartz gangue. Quartz vein textures at T4 Hill are dominated by primary and
recrystallisation textures. Quartz forms the dominant phase with minor chalcedony.
Lattice-bladed quartz is closely associated with sulphides thus suggesting ore-bearing
fluids precipitated either barite or calcite.
Quartz vein textures in the Gift prospect vary significantly between mineralised and
unmineralised veins. Unmineralised veins share many similarities with the veins at
Central Ridge such as the predominance of chalcedony crustiform colloform banded
Chapter 4 Gold-bearing quartz veins
veins and the absence of sulphides. Mineralised veins exhibit primary and replacement
similar to Conical Hill and T1 Hill.
Table 4.3 Summary of primary recrystallisation and replacement quartz textures for ConicalHill, Central Ridge, T1 Hill, Gift prospect, and T4 Hill, Wang Yai, central Thailand.
81
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Texture Type
Conical Hill
Central Ridge
T1 Hill
Gift Prospect
T4 Hill
Primary Massive *** ** ** Crustiform * *** *
** Colloform * *** * ** Cockade ** ** * * Moss *** ** * Comb ** * * *** ** Zonal * ** * Recrystallisation
Mosaic ** ** * * ** Feathery ** Flamboyant ** * ** Ghost-sphere ** * ** Replacement
Lattice-bladed * *** ** * Ghost-bladed ** * Parallel-bladed Pseudo-acicular *** Saccharoidal ** * **
*rare **common ***dominant
Gangue Mineralogy
Each vein system at Wang Yai has its own gangue mineralogy assemblage. Due to
intense weathering and silica replacement of original gangue mineralogy most gangue
assemblages have been determined from the texture, and morphology of silica
replacement textures.
Chapter 4 Gold-bearing quartz veins
82
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Quartz is the major gangue mineral at Wang Yai and occurs in all vein systems.
Pervasive silica replacement textures indicating a calcite, adularia, or barite precursor are
present in only some vein systems. These systems include Conical Hill, T1 Hill, T4 Hill
and mineralised veins in the Gift prospect. The replacement textures are indicative of
calcite and adularia precursors and are generally associated with mineralisation. Barite
pseudomorphs occur only in the Gift prospect although barite was found in host rock
replacing plagioclase phenocrysts proximal to the Central Ridge vein. Replacement
textures are absent or less extensive for the chalcedony dominated veins of Central Ridge
and Gift (B, C, D, and E veins) but chlorite is sometimes preserved with opaline silica
bands.
The association of calcite and adularia with mineralisation is quite significant in terms of
constraining the mechanism of ore deposition. This has further implications with regard
to exploration in that the identification of replacement textures in vein systems can assist
in determining barren veins from mineralised veins.
Mineralisation
Precious metal mineralisation is best developed at Conical Hill, where grains of electrum
occur within ginguro ore bands dominated by argentite usually associated with fine
grained quartz, silica pseudomorphs of adularia, and replacement textures. At T1 Hill
ginguro ore bands also occur in conjunction with microcrystalline quartz and replacement
textures. Sulphides assemblages at T1 Hill differ from Conical Hill in that rather than
argentite and electrum assemblages, pyrite and chalcopyrite form the dominant
assemblages. Similarly at Gift prospect mineralised zones exhibit pyrite and chalcopyrite
and not argentite.
Mineralisation at Wang Yai is characterized by the paucity of base metals. Considering
that current epithermal models (e.g Buchanan, 1981) show a vertical zonation of precious
metals occurring above base metals, it is likely that the current surface at Wang Yai
represents the upper part of a larger hydrothermal system.
Chapter 4 Gold-bearing quartz veins
83
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
4.9 Interpretation
There are many descriptive zonation models for epithermal low-sulphidation deposits.
This study will reference the model proposed by Buchanan (1981) which is a summary of
60 epithermal deposits hosted in volcanics in western USA. This model illustrates the
ore, gangue, and alteration zones in epithermal veins and is widely used in the current
literature. More recently Morrison et al. (1990) has developed a zoning model of quartz
textures in epithermal deposits. For this study the model developed by Morrison et al.
(1990) will be superimposed on the model by Buchanan (1981) (See Figure 4.19).
The aim of this interpretation is to use the observations of quartz textures, gangue
mineralogy, and ore mineralogy of the Wang Yai prospects to determine the relative
depth of formation for each system.
Conical Hill
In the main vein at Conical Hill, four vein types have been recognised and of these A-
type (milky white quartz chalcedony crustiform colloform) and C-type (massive sugary
and saccharoidal quartz) veins that host ‘ginguro’ sulfide-rich mineralization
assemblages. Quartz textures are predominantly primary and replacement.
Mineralisation preferentially occurs adjacent to replacement textures such as adularia
pseudomorphs, bladed-calcite, microcrystalline quartz, saccharoidal quartz, and pseudo-
acicular textures. Ore minerals occur in ginguro bands and comprise of electrum and
argentite. Electrum can occur as free grains or within cracks of argentite. Minor pyrite
and chalcopyrite also occur.
84
Figure 4.19 Epithermal model developed by Buchanan, (1981) showing the zonation of ore and gangue mineralogy as a function of depth and temperature. Morrison et al., (1991) quartz textural classification scheme and interpretated positions of Wang Yai vein systems are superimposed the Buchanan, (1981) model.
Chapter 4 Gold-bearing quartz veins
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 4 Gold-bearing quartz veins
85
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
Based on mineralisation styles, gangue, ore mineralogy and quartz textures Conical Hill
is interpreted to have formed at the precious metal interval slightly above or at the zone
of boiling. This zone is located approximately 200 m-300 m below the palaeo-surface
and is the position whereby precious metal deposition occurs in response to boiling
(Buchanan, 1981). Conical Hill exhibits many of factors that are consistent with this
level of formation. This interpretation is based on the evidence below.
• The predominance of primary quartz textures such as massive, moss, crustiform
and crystalline quartz is consistent with the lower part of the crustiform colloform
zone (Morrison et al., 1991).
• The predominance of argentite and electrum and absence of base metal sulphides
is consistent with formation at the precious metal ore interval (Buchanan, 1981).
• The presence of vein adularia and bladed calcite strongly suggests a boiling event
(Simmons et al., 2000) and this is consistent with the precious metal ore interval
(Buchanan, 1981).
• Upon initiation of boiling, hydrothermal fluid becomes alkaline which allows for
the precipitation of adularia, and CO2 loss occurs allowing for the precipitation of
calcite (Dong et al., 1995). Adularia and calcite both occur at Conical Hill.
• The presence of rhombic adularia suggests that formation was formed under rapid
crystallisation conditions such as boiling of hydrothermal fluids (Dong and
Morrison, 1995).
Central Ridge
Three dominant vein types occur at Central Ridge. All vein types are characterized by
poor gold grade and the predominance of chalcedony rather than quartz in well developed
crustiform colloform vein stages. In contrast to Conical Hill, Central Ridge quartz
textures are dominated by primary and recrystallisation textures rather than replacement
Chapter 4 Gold-bearing quartz veins
86
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
textures. Calcite and adularia pseudomorphs are absent and the only sulphides observed
were very minor weathered pyrite and chalcopyrite grains preferentially occurring within
fine grained crystalline quartz horizons of crustiform colloform veins.
Based on the observations of quartz textures, gangue mineralogy and ore mineralogy
Central Ridge is interpreted to have formed at a lower temperature than Conical Hill. A
shallower level of formation than Conical Hill, possibly within the upper 100-150 m of
the vertical interval is proposed for Central Ridge. However it is possible that they
formed at the same level but at different temperatures. The evidence to suggest lower
temperature hydrothermal fluids or shallow level of formation are based on quartz,
gangue and ore mineralogy assemblages and include the following;
• Primary chalcedony and opal and their associated recrystallisation textures at
Central Ridge are possibly inherited from an amorphous silica gel precursor
(Dong and Morrison, 1995). Henley and Ellis (1983) state that amorphous silica
is predominant at high elevations in epithermal fields and indicates a temperature
of up to 150˚C.
• To precipitate amorphous silica the fluids should be slightly saturated with silica
and temperatures between 100˚C and 190˚C (Fournier, 1985; Henley and Ellis,
1983).
• Gold grade is characteristically low and only occurs in samples which exhibit
pyrite grains which suggests that either electrum was not present in cut thin
sections of forms as inclusions within pyrite grains. Electrum occurring as
inclusions in pyrite is consistent with the upper 100 metres of the model proposed
by Buchanan (1981).
Chapter 4 Gold-bearing quartz veins
87
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
T1 Hill
The availability of drill core at T1 Hill allows the assessment of quartz textures, gangue
mineralogy and ore mineralogy over a vertical interval of up to 250m. Four main vein
stages can be identified from surface samples at T1 Hill. Mineralised stages are
characterized by primary quartz textures such as milky white chalcedony crustiform
bands, ‘smoky’ grey crystalline quartz and moss textures. Replacement textures after
calcite and/or adularia are common and include lattice bladed and pseudoacicular quartz.
Sulphides are associated with micro crystalline quartz horizons and replacement textures.
In drill core mineralisation is also associated with replacement textures in ‘smoky’ green
chlorite quartz veins. Ore mineralogy is dominated by pyrite and chalcopyrite grains
occurring within the wall rock and to a lesser extent in veins. In surface samples pyrite
and chalcopyrite preferentially occur together in thin bands. Pyrite and chalcopyrite in
drill core are more abundant than in surface samples. Most pyrite preferentially occurs in
wall rock and chalcopyrite in quartz carbonate veins.
It is interpreted that like Conical Hill the level of formation is deeper than that of Central
Ridge and could potentially be deeper than Conical Hill. Similar to Conical Hill boiling
is thought to be closely associated with ore bearing phases.
• Zonation of increasing quartz and decreasing chalcedony with depth is observed
at T1 Hill. This zonation is a typical feature of epithermal deposits and has been
documented at Pajinjo (Bobis et al., 1995), McLaughlin (Sherlock et al., 1995),
Cracow (Dong and Zhou, 1996), Comstock (Hudson, 2003), and Guanajuato
(Buchanan, 1981).
• In drill core, vein textures are dominated by comb quartz carbonate chlorite veins.
The presence of comb quartz indicates that the fluids were slightly saturated with
respect to quartz which requires slow cooling at temperatures between about
200˚C and 340˚C (Fournier, 1985). This temperature is much higher than the
interpreted temperature between 100˚C and 180˚C for Central Ridge.
Chapter 4 Gold-bearing quartz veins
88
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
• Crystalline quartz and chalcedony in surface samples represent temperatures of
between 150˚C and 200˚C and an amorphous silica precursor (Dong et al., 1995).
This is lower than temperatures inferred to occur in drill hole located veins. Late
stage comb quartz veins resembling veins at depth cross-cut quartz and
chalcedony surface samples. This comb quartz stage is inferred to be a high
temperature stage.
• Replacement textures, adularia rhombs, and bladed calcite associated with
sulphides suggests that boiling occurred during ore deposition.
• The breccia stage in surface vein float suggests that episodic hydraulic fracturing
occurred which is probably a result of quartz and/or calcite precipitation sealing
fluid channel conduits (De Ronde and Blattner, 1988; Fournier, 1985). Episodic
fracturing caused by imbalances of confining pressures may create conditions that
are conducive to the precipitation of precious metals (Fournier, 1985). These
conditions include increase in permeability, decrease in pressure, partitioning of
H2S and CO2 into a steam phase (steam = ↑ ph; Fournier, 1985).
Gift Prospect
The Gift prospect comprises of 5 vein systems that differ in terms of mineralogy, quartz
textures and gold grade. Quartz textures associated with high grade A and F systems
include primary massive, crystalline, moss, cockade and comb quartz. Replacement
textures including ghost bladed, lattice bladed, saccharoidal and quartz pseudomorphs
after adularia are also common. Low grade systems (ie B, C, D, E) exhibit primary
quartz textures dominated by chalcedony crustiform colloform banded veins. Breccia
veins also occur and comprise of unmineralised chalcedony crustiform colloform vein
clasts in a chalcedony dominated matrix.
Based on gold grade, vein systems at Gift can be divided into barren and mineralised
veins. Mineralised veins (A and F) share similar quartz textures, gangue mineralogy, and
Chapter 4 Gold-bearing quartz veins
89
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
ore mineralogy and are interpreted to have formed at around the same vertical interval or
from similar fluids and temperatures. This interval is interpreted to be at or slightly
above the zone of precious metal deposition (200-250 metres). This classification is
largely based on quartz textural analysis and gangue mineralogy as most sulphide
assemblages have weathered to limonite and goethite. In contrast to mineralised systems,
barren systems are characterized by the predominance of chalcedony + quartz ± barite
crustiform colloform banded veins which, display textural similarities to Central Ridge
veins. It is interpreted that these veins form at shallow levels or at lower temperatures
than the mineralised veins.
Evidence suggesting formation at or slightly above the precious metal zone for
mineralised veins includes:
• Quartz textures are dominated by crystalline quartz and only minor chalcedony
phases. This is consistent with the quartz textural zonation model of Morrison et
al. (1990) at the precious metal interval. Ore mineralogy including pyrite,
chalcopyrite, and rare electrum are typical assemblages of the precious metal ore
interval (Buchanan, 1981).
• Gangue mineralogy such as crystalline quartz, calcite, and adularia is indicative of
formation just above the precious metal zone (200-250 metres; Buchanan, 1981).
• Replacement textures generally associated with ore mineralogy such as bladed
quartz and pseudoacicular quartz indicates that during ore deposition boiling
provided favourable conditions for the precipitation of adularia and calcite.
• Absence of precious metals and predominance of pyrite and minor chalcopyrite
suggest that formation occurred above the precious metal interval.
Evidence to suggest barren veins formed at shallower levels (100 – 200 metres) than
mineralised veins includes.
Chapter 4 Gold-bearing quartz veins
90
Geological setting, nature of mineralization and fluid characteristics of the Wang Yai prospects, central Thailand
• Predominance of chalcedony in vein stages suggest that fluids were saturated in
respect to quartz and were at temperatures of 180ºC or below (Fournier, 1985).
According to Buchanan (1981) and Hedenquist (2000) temperatures of around
180ºC are likely to occur at shallow levels between 180 and 90 metres.
• Gangue mineralogy assemblages including chalcedony, calcite, and barite are
consistent with formation at 200 – 100 metres below the surface (Buchanan,
1981).
Chapter 5 Fluid Characteristics
Chapter 5 Fluid Characteristics
Stable isotopes (oxygen and sulphur) and fluid inclusions have been used to help better
constrain the fluid characteristics responsible for mineralisation at Wang Yai. This
chapter will present a description of results followed by a discussion for the implications
of the results.
5.1 Oxygen Isotopes
δ18O values of vein quartz can be used in epithermal systems to characterize the
hydrothermal fluids, identify the source of water in the system, and to estimate the degree
of meteoric water-rock exchange (John et al., 2003). Studies such as these may also
provide important insights into ore forming processes. In this study an isotopic study of
vein quartz was undertaken in an attempt to understand the source of fluids and the
relationship between oxygen isotope, gold grade and vein textures at Wang Yai. The
broad aim of this research was to establish a useful exploration tool that could vector
towards high grade parts of the system.
Methods
A total of 16 vein samples were selected from vein systems at Wang Yai. The silica
polymorphs in samples ranged from crystalline quartz to chalcedony to opaline quartz.
All samples displayed a variation in gold grade (from 0.00 to 36.5 g/t Au). The samples
selected include two from Conical Hill, four from Central Ridge, three from T1 Hill, one
from T4 Hill, five from the Gift prospect (see Table 5.1).
The quartz selected for δ18O analyses was analysed using conventional techniques and a
Micromass 602E mass spectrometer. δ18O values are given in per mil relative to Vienna
Standard Mean Ocean Water (SMOW). Analytical precision is ±0.2 per mil (one standard
deviation). Measured isotopic values were normalised against an international standard
through the repeat analysis of NSB 28 (+9.6 ‰ δ18O).
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
Table 5.1 Summary of δ18O (SMOW) values for quartz at Wang Yai, Central, Thailand
Sample no. Material analysed Occurrence δ18O (SMOW) Location Description Assay g/t Au
ER017966
ER017427
ER017439
ER017244
ER017244
ER016951
ER016934
ER016905
ER016925
ER016926
ER017453
ER017511
ER017507
ER017509
ER017499
ER017464
Crystalline quartz
Crystalline quartz
Opaline silica
Opaline silica
Opaline silica
Chalcedony, opal
Chalcedony, opal
Crystalline quartz
Quartz, chalcedony
Quartz, chalcedony
Crystalline quartz
Crystalline quartz
Chalcedony
Chalcedony
Chalcedony, quartz
Quartz
Massive vein
Massive vein
Banded vein
Vein
Vein
Vein
Vein
Vein
Vein breccia
Vein breccia
Vein
Vein breccia
Banded vein
Banded vein
Banded vein
Massive vein
13.5
13.0
17.5
16.5
16.2
14.0
14.5
12.2
12.4
13.3
13.3
11.0
14.6
14.8
15.5
12.5
Conical Hill
Conical Hill
Central Ridge
Central Ridge
Central Ridge
Central Ridge
Central Ridge
T1 Hill
T1 Hill
T1 Hill
T4 Hill
S.V Prospect
Gift Prospect
Gift Prospect
Gift Prospect
Gift Prospect
Milky white, crystalline quartz vein
Crystalline quartz with ginguro ore
Banded opaline silica + chlorite vein
Opaline silica-chlorite banded vein
Opaline silica-chlorite banded vein
Crustiform colloform banded
Crustiform colloform banded
Crystalline quartz with cockade textures
Massive crystalline quartz + chalcedony
Grey crystalline quartz + chalcedony clasts
Poorly developed crustiform colloform bands
Brecciated crystalline quartz vein
Pink recrystallised chalcedony
Pink chalcedony crustiform colloform bands
Banded chalcedony vein with comb quartz
Massive grey milky quartz vein
6.3
36.5
0.00
0.00
0.00
0.00
0.4
2.0
0.7
3.8
0.1
0.1
0.0
0.0
0.0
2.8
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
5.1.1 Results
Τhe data of δ18O values are spatially shown in Figure 5.1. δ18O values show a range from
+11 to +17.5 per mil. δ18O values broadly correlate with variation of gold grade and vein
textures. For example, low δ18O values are associated with high gold grade and textures
dominated by crystalline quartz. These vein systems (Conical Hill, T1 Hill, T4 Hill and
‘D’ Zone at Gift) have average δ18O values of between +12 and +13.5 per mil. The S.V
prospect which is distant from the main vein zones at Wang Yai has a value of +11 per
mil. High δ18O values correlate with low or no gold grade and opaline silica-chalcedony
dominated quartz textures. For example Central Ridge and B, C, D and E vein systems at
Gift show high (+14.6 – +17.5 per mil) oxygen isotope values. These systems are
characterised by unmineralised chalcedony ± opaline quartz veins. At Central Ridge
δ18O values show an excellent relationship of zoning with vein textures and gold grade.
This zonation is characterised by high δ18O values (+17.5, +16.5, +16.2 per mil)
occurring at the southern end of the system where opaline quartz phases predominant and
gold grade is negligible. At the northern end of the vein δ18O values are lower (+14.0 ,
+14.5 per mil) corresponding with an increase in gold grade.
Quartz vein δ18O values for vein systems in the Gift prospect also correlate with vein
textures and mineralogy. Vein systems B, C, D and E which are dominated by
chalcedony crustiform colloform banded veins and negligible gold grade (i.e; <0.01 g/t
Au) exhibit values of 14.6 and 14.8 per mil. In contrast, vein system D which is
dominated by crystalline quartz and average gold grade >2.0g/t Au shows values of 12.5
per mil.
Calculated oxygen isotope compositions of ore fluids
Oxygen isotope compositions of ore fluids were calculated using quartz-water
fractionation data from Zheng (1993) (Equation 5.1). Calculated isotopic compositions
of ore fluids for 180ºC, 250ºC, and 300ºC are summarised in Table 5.2.
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Equation 5.1. Quartz-water fractionation equation (Zheng 1993) D: 4.480; E: -4.77; F: 171 (from http://www.ggl.ulaval.ca/cgi-bin/isotope/isotope4alpha.cgi
Chapter 5 Fluid Characteristics
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 5.1 Aerial photo of Wang Yai showing the raw δ18O SMOW values in per mil from each vein system (in yellow and black). Bright colours (red, orange, pink) represent low δ18O SMOW values and dark colours represent (blue) high oxygen isotope values. Corresponding gold grade is labelled for each δ18Ovalue. Yellow represents gold grade and white no gold grade.
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Chapter 5 Fluid Characteristics
Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Table 5.2 Summary of isotopic compositions of ore fluids for 180ºC, 250ºC, and 300ºC calculated from quartz-water fractionation Zheng (1993)
Location/ Sample no
Assay g/t Au
δ18O (SMOW)
δ18O180˚C δ18O250˚C δ18O300˚C Vein Descriptions
Conical Hill ER017966 6.3 13.5 0.5 4.5 6.5 Milky white, crystalline quartz vein ER017427 36.5 13 0 4 6 Crystalline quartz with ginguro Central Ridge ER017439 0 17.5 4 8.8 10.5 Banded opaline silica + chlorite ER017244 0 16.5 3 7.5 9.5 Opaline silica-chlorite banded vein ER017244 0 16.2 3.2 7.2 9.2 Opaline silica-chlorite banded vein ER016951 0 14 1 5 7 Crustiform colloform banded ER016934 0.4 14.5 1.5 5.5 7.5 Crustiform colloform banded T1 Hill ER016905 2 12.2 -0.8 3.2 5.2 Crystalline quartz with cockade textures ER016925 0.7 12.4 -0.6 3.4 5.4 Massive crystalline quartz + chalcedony ER016926 3.8 13.3 0.3 4.3 6.3 Grey crystalline quartz + chalcedony clasts T4 Hill ER017453 0.1 13.3 0.3 4.3 6.3 Poorly developed crustiform colloform bands S.V ER017511 0.1 11 -0.2 2 4 Brecciated crystalline quartz vein Gift Prospect ER017507 0 14.6 1.6 5.6 7.6 Pink recrystallised chalcedony ER017509 0 14.8 1.8 5.8 7.8 Pink chalcedony crustiform colloform bands ER017499 0 15.5 2.5 6.5 8.5 Banded chalcedony vein with comb quartz
Massive grey milky quartz vein 5.5
3.5 ER017464 2.8 12.5 -0.5
Chapter 5 Fluid Characteristics
Table 5.2 shows that calculated oxygen isotope compositions for 180˚C yielded values
between 1.0 and 4.0 per mil for poorly mineralised chalcedony dominated veins (i.e;
Central Ridge, B, C, D, and E Gift) and -0.8 to 0.5 per mil for well mineralised,
crystalline quartz veins. Oxygen isotope compositions at 250˚C yielded values between
8.0 and 5.0 per mil for poorly mineralised veins and between 2.0 and 4.5 per mil for well
mineralised veins. At 300˚C, oxygen isotope compositions yielded values between 7 and
10.5 per mil for poorly mineralised veins and between 2 and 4.5 per mil for well
mineralised veins. Figure 5.2 shows a histogram of the calculated oxygen isotopic
compositions of ore fluids.
0
1
2
3
4
5
6
7
8
9
10
-2 -1 0 1 2 3 4 5 6 7 8 9 10 11
Freq
uenc
y
300˚C250˚C180˚C
magmatic water compositionsmeteoric water compositions
δ18O SMOW (per mil)
Figure 5.2 Frequency histogram of calculated δ18O SMOW of ore fluids at 180˚C, 250˚C, and 300˚C using quartz fractionation data from Zheng (1993). Note the compositions of magmatic water and meteoric water in black (Rollinson, 1993).)
The isotopic compositions show a shift towards depleted compositions with declining
model temperatures. This shift corresponds/overlaps with typical meteoric fluid
composition. At higher model temperatures (300ºC), δ18O compositions are more
magmatic. This may be attributed to four factors including; water rock interaction, fluid
mixing, finite reservoir effects, and boiling. Each of these factors is discussed in more
detail at the end of the chapter.
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
5.2 Sulphur isotopes
Stable δS34 isotope analyses of wallrock and vein located pyrite were undertaken to
determine the source of sulphur at Wang Yai. For this study three samples with vein or
wallrock disseminated pyrite were selected for laser ablation at the Central Science
Laboratory, University of Tasmania. One sample (ER017459) was selected from the Gift
prospect and two were from T1 hill drill hole WYRD016.
The results from laser ablation of pyrite (summarised in Table 5.3) show that vein located
pyrite yields δ34S per mil values between +1.59 and -0.09 per mil. Wallrock located
pyrite yields δ34S values between +3.82 and -0.04 per mil. There is a broad trend of wall
rock pyrite assuming heavier δ34S values.
Table 5.3 Summary of δ34S values for vein and wallrock located pyrite, Wang Yai, central Thailand Sample no Location Number spots ablated δ34S ‰ ER017459 (Gift) Vein 2 +1.56 ER017459 (Gift) Vein 1 +0.46 WYRD016@240m Vein 1 +1.59 WYRD016@240m Vein 2 -0.09 WYRD016@240m Wallrock 2 -0.04
WYRD016@240m Wallrock 2 +2.39 WYRD016@243.7m Wallrock 3 +3.82 WYRD016@243.7m Wallrock 4 +0.58 WYRD016@243.7m Vein 1 +0.03
0
1
2
3
4
-1.0 0.0 +1.0 +2.0 +3.0 +4.0 +5.0
Freq
uenc
y
Wall rock pyriteVein pyrite
δ 34S ‰
δ 34S ‰ values of vein pyrite
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Figure 5.3. Histogram of δ34S values for wall rock and vein located pyrite.
Chapter 5 Fluid Characteristics
5.3 Fluid inclusion studies
Fluid inclusion studies was carried out to understand the thermal aspect of ore deposition
and to constrain the variation of oxygen isotopic composition. Linkam TH600
heating/freezing stage was used in this study. The general method and procedure for
heating/freezing experiments are reported elsewhere (Roedder, 1984). The precision of
the temperature measurements is better than +1oC for heating and +0.3oC for freezing.
Accuracy of the measurements was insured by calibration against synthetic fluid
inclusions and the observed triple point of CO2 (-56.6oC), the freezing point of water
(0.0oC), and the critical point of water (374.1oC). Fluid inclusions were classified in a
temporal sense as primary, secondary and pseudosecondary relative to the time of
trapping as defined by Roedder (1984). The fluid inclusion characteristics of the Wang
Yai prospects are shown in Figure 5.4
Fluid inclusions from the Wang Yai prospect can be classified into two major types based
on phases observed in the inclusions at room temperature. They include:
Type I: Two-phase, liquid and vapor inclusions. Type I inclusions are isolated, and occur
away from healed microfractures. They are primary inclusions as their formation can be
related to the growth zones of the host quartz (Figure 5.4A). In this study, only primary
Type I inclusions are studied.
Type II: Two-phase, liquid-rich inclusions with a variable liquid and vapor ratio. Their
irregular shape with variable liquid and vapor ratio indicates a secondary origin. The
randomly distributed array of these secondary Type II inclusions and their textural
features possibly suggest post-deformational shearing during their emplacement.
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
gu C
A
Primary FI
B
B
C D Primary
262.6oC Ice melting -5.1 Salinity 8.14 wt % NaCl equiv.
Secondary FI D
Fi re 5.4 A) Photomicrographs showing Types I and II fluid inclusions in euhedral quartz (sample ER016916) B) lose-up of two primary inclusions (sample ER016916). C). Close up of two primary inclusions (sample ER016916). D) Close up of primary Type I inclusions.
5.3.1 Homgenisation temperatures
Homogenisation data for Type I inclusions from Wang Yai are shown in Table 5.4. A
total of nineteen homogenisation temperatures were measured for three samples.
Measured homogenisation temperatures display a range of 134 to 298.4˚C. Primary fluid
inclusions from sample ER016916 (0.85 g/t Au) show a range in temperature of 218 to
298.4˚C, all but one occur between 240 and 290˚C. The anomalous value of 218˚C may
be attributed to analyses of a secondary inclusion (consistent with other low temperatures
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
measured elsewhere). Fluid inclusions from this sample were analysed from euhedral
quartz that infills open spaces in crystalline quartz. Primary fluid inclusions for sample
ER016925 show lower (180 to 218˚C) homogenisation temperatures than sample
ER016916. Fluid inclusions from ER016925 were analysed from grey crystalline matrix
quartz that is host to disseminated pyrite and electrum (0.86 g/t Au). δ18O values for this
sample are +12.4 per mil. Sample ER017499 is from Gift prospect vein system C (Figure
4.1) and yields homogenisation temperatures of 141 to 210˚C. Fluid inclusions were
analysed from a band of comb quartz with well developed euhedral crystals and growth
zones. This sample yields the lowest temperatures for all three samples and the lowest
gold grade (below detection). δ18O values for this sample are +15.4 per mil. From this
data set δ18O values show a relationship with homogenisation temperature. δ18O values
show high values for low homogenisation temperatures and lower δ18O values for high
homogenisation temperatures. This implies that temperature is not solely controlling
grade, other processes such as water rock interaction and boiling maybe responsible.
5.3.2 Salinity
The salinity values for the three samples range from 1.1 to 8.1 wt % NaCl equivalent.
However most values are between 3 and 5 wt % NaCl equivalent. The salinity values do
not show a spatial zonation or a correlation with gold grade. All samples lacked evidence
of boiling textures. Salinity values for sample ER016916 (comb quartz) show a range of
2.1 to 8.1 wt % NaCl equivalent. Sample ER016925 (crystalline quartz) which is from
the same prospect (T1 Hill) yields values of between 3.4 and 5.0 wt % NaCl equivalent
and sample ER017499 from the C vein system at Gift yields values between 1.1 and 5.7
wt % NaCl equivalent.
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
Table 5.4. Summary of fluid inclusion data, Wang Yai, central Thailand
Location/Sample No./ Quartz texture/av.δ18O value
Mineral studied
Inclusion type
Th (L-V)-L
Tm-ice wt % NaCl equiv*
T1 Hill ER 016916 Quartz I 253.0 Euhedral well developed I 264.0 comb quartz I 296.0 -2.8 4.7 I 262.6 -5.1 8.1 I 264.7 -4.5 7.2 I 218.6 -2.0 3.4 I 283.3 -2.5 4.2 I 298.4 -1.2 2.1 I 237.0 -2.8 4.7 T1 Hill ER 016925 Quartz I 134.7 -3.1 5.0 Grey crystalline quartz I 187.8 -2.0 3.4 +12.4 δ18O I 213.4 -2.0 3.4 I 218.8 C vein system (Gift) ER 017499 Quartz I 152.5 Comb quartz I 163.7 +15.4 δ18O I 203.0 I 215.7 -1.8 3.1 I 141.2 -0.6 1.1 I 185.4 -3.5 5.7
* Salinities were calculated using the last ice melting temperature (wt % NaCl equiv.) and equation of Bodnar (1993)
5.4 Discussion
5.4.1 Oxygen Isotopes
Quartz oxygen isotopic values show a clear correlation with vein textures and gold grade
at Wang Yai (as outlined above). Well mineralised crystalline quartz dominated veins
(i.e, Conical Hill, T1 Hill, T4 Hill, A and F Gift systems) show low δ18O values (12 to 13
per mil). Unmineralised chalcedony dominated veins display high δ18O values (14.5 – 17
per mil).
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
The histogram of calculated δ18O SMOW values for fluid temperatures at 180˚C, 250˚C,
and 300˚C using quartz water fraction factor of Zheng (1993), shows that there is a shift
to lower δ18O isotopic compositions with decreasing temperature. For example, at Central
Ridge and Gift the calculated fluid composition responsible for chalcedony (formed
<200˚C) can be a few per mil lighter than fluid responsible for the earlier high
temperature (>200˚C) quartz (see Table 5.2) There are four plausible processes that can
be attributed to the shift to depleted oxygen isotope values. They include water rock
interaction, fluid mixing, finite reservoir effects and boiling.
Water-Rock interaction: Water rock interaction is inherited during hydrothermal
alteration. When hydrothermal fluids infiltrate a portion of rock there, is an inherent shift
in isotopic composition of the fluid that will directly influence the composition of
subsequent precipitating minerals (Taylor, 1974). For example if a hydrothermal fluid
interacts with wall rock that has a higher isotopic composition than the fluid then mass-
balance effects will result in the wall rock assuming heavier isotopic values and lighter
fluid values. The degree at which the fluid composition shifts, is dependant on
temperature (i.e; larger shift at low temps and smaller shift at high temps), and the water/
rock ratio. The processes of isotopic exchange during fluid interaction can be explained
using the model from Harris et al. (2005). This model is calculated for temperatures of
300˚C (which are only slightly above the highest homogenisation temperatures at Wang
Yai) and magmatic water with compositions between 5.7 and 9.8 per mil. Therefore
assuming an average volcanic wallrock composition of +8 per mil and an unevolved
magmatic fluid of +7 per mil water rock exchange will result in the enrichment of δ18O in
wall rock (up to 4 per mil) and depletion of δ18O in fluids (Harris et al., 2005). This shift
in δ18O composition of fluid buffered against rock, becomes more pronounced at lower
temperature (implicit in the fractionation factors used to model fluid compositions).
Therefore the shift to depleted δ18O values shown in Figure 5.2 may represent the
presence of an evolved fluid that has interacted with wall rock.
Finite reservoirs effects: Finite reservoir effects can help explain isotopic variation in a
hydrothermal system (Pollard et al., 1991). This process can be linked with water rock
interaction. Finite reservoir processes is a consequence of hydrothermal fluids moving - 102 -
Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
through conduits to exchange heavy δ18O values from the liquid to quartz as it is
precipitated. This results in an imbalance of δ18O values between precipitated substrate
and fluids. Subsequently through simple mass balancing between quartz substrate and
fluids the isotopic composition of fluids become lighter.
Boiling: Boiling is a process that may also be responsible for the shift in δ18O values. At
Wang Yai there is evidence of boiling in gold bearing veins (see Chapter 4 section 4.9).
However it has been documented in many papers that boiling alone is unlikely to cause
significant shifts in isotopic compositions. For example at Hishikari, Japan Hayashi et al.
(2001) found that although bladed quartz (indicative of boiling) is associated with a large
shift of δ18O values (+6 per mil to 12 per mil) 92 % of the fluid must boil before this shift
can occur. Hayashi et al. (2001) concluded that vein formation is unlikely during
extreme boiling, therefore the shift in δ18O values is attributed to fluid mixing occurring
concomitant with boiling.
Fluid mixing: Fluid mixing of magmatic fluid and meteoric fluid may also be attributed
to the shift in calculated δ18O values although due to the limited data set and unknown δD
fluid values constraining the evolution fluid mixing is quite problematic and beyond the
scope of this project.
Of the four processes listed above it is likely that water rock interaction and finite
reserviour effects are considered important the shift in δ18O values. To help better
constrain the degree of water rock interaction and fluid mixing δ18O and δD values of
host rock and associated hydrothermal alteration assemblages needs to be determined.
5.4.2 Sulphur Isotopes
δS34 isotopic values of pyrite from gold bearing veins and alteration assemblages at Wang
Yai occur close to zero per mil. This suggests that a magmatic source of sulphur that may
have come from at depth or from volcanic host rocks through which the fluid passed
(Heald et al., 1987).
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
5.4.3 Fluid Inclusions
Primary fluid inclusion data from the samples analysed shows homogenisation
temperatures are consistent with typical epithermal low sulphidation systems (Simmons
et al., 2005). At Wang Yai homogenisation temperatures display a relationship with vein
textures and gold grade. Mineralised crystalline quartz veins at T1 Hill show lower
temperatures (180 to 218˚C) than late stage unmineralised comb quartz (ER016916) (218
to 298.4˚C). Based on the relationship between mineralisation and homogenisation
temperatures of 180 to 218˚C it is interpreted that ore bearing fluids were at temperatures
from 200 to 250˚C during the deposition phase. This temperature range is consistent with
the 200 to 300˚C range for 16 low sulphidation epithermal deposits in North and South
America (Heald et al. 1987). Moreover, it is consistent with textural evidence and the
preservation of chalcedony (implying temperatures <200˚C) and boiling (e.g., adularia).
The high and diverse range of temperatures for primary inclusions in sample ER016916
could indicate that during the ore depositional event boiling occurred (Bodnar et al.,
1985).
The unmineralised chalcedony dominated veins at system C in the Gift prospect show
low temperatures between 140 and 200˚C. This temperature falls within the bracketed
temperature of chalcedony formation (<180˚C; Fournier, 1985). These veins maybe
offshoots off the main vein system and appear to have formed at low temperatures.
Heald et al. (1987) in their study of North and South American low sulphidation
epithermal deposits highlights the presence of late stage, veins depositing gangue
mineralogy at temperatures between 140˚C and 200˚C. These temperatures are consistent
with the homogenisation temperatures of barren chalcedony dominated veins at system C
in the Gift prospect.
Calculated salinity values show an average range of 3-6wt% NaCl equivalent. These
values are slightly higher than values for typical gold dominated epithermal deposits
hosted in andesitic rocks. Most gold dominated deposits have average weight percent
NaCl values of around 1.6 to 2.0 wt % NaCl equivalent and up to 5 wt % equivalent for
Au-Ag systems (Hedenquist and Henley, 1985; Sherlock et al., 1995; Simmons et al.,
2005). Salinities up to 20wt % are found in some quartz carbonate veins in low - 104 -
Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 5 Fluid Characteristics
sulphidation systems (e.g 10 wt % NaCl equiv. at Creede, Barton et al., 1977) but these
systems are Ag-Pb-Zn rich (not like Wang Yai). As the salinity values at Wang Yai are
higher than that of other epithermal deposits it is possible that the presence of CO2 vapor
in fluid inclusions played a role in contributing to freezing point depression (i.e,
increasing the salinity values; Hedenquist and Henley, 1985). If CO2 vapour has
contributed to the freezing point depression then it is possible that the salinity values are
not representative of the original quartz precipitating fluids. Alternatively, the higher
salinity of the fluids associated with mineralization may imply a small component of
magmatic fluid introduced to the Wang Yai system. This requires further validation.
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Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 6 Conclusions
106 Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Chapter 6 Conclusions
6.1 Geological setting
Geological mapping at Wang Yai has revealed gold mineralisation is hosted in
volcanogenic sandstone, breccia and conglomerate which are intruded by coherent
feldspar phyric andesite. Minor lithological units include mudstone and thinly bedded
fossiliferous limestone. The succession is cut by diorite dykes.
LA-ICP-MS U-Pb ziron studies of quartz phyric rhyodacite and quartz breccia/sandstone
have yielded ages of 321 ± 5 Ma and 247 ± 4 Ma respectively. These ages suggest that
basement felsic volcanics (rhyodacite) are overlain by Permian to Triassic volcanic
stratigraphy.
The host rock geochemical characteristics at Wang Yai yield island arc and continental
arc affinities. Older rocks including andesitic lithic breccia and andesitic sandstone have
island arc affinities. This is consistent with palaeo reconstructions of the Permian which
include island arc volcanism associated with subduction of the Shan-Thai and Indochina
cratons (Dedenczuk, 1998). Younger intrusive rocks (andesite and diorite) yield
continental arc affinities.
6.2 Nature of mineralisation
The volcanic succession at Wang Yai hosts eleven vein systems. The systems occur in
the following prospects; Conical Hill, Central Ridge, T1 Hill, T4 Hill, S.V and Gift.
Vein texture and mineralogy studies have identified zonation between gold-bearing vein
systems and barren systems. Using current epithermal models this zonation has been
explained through depth related zonation due to boiling. At shallow levels in the system
Central Ridge, and Gift prospect vein systems B, C, D, and E show chalcedony and
opaline quartz crustiform colloform veins which are indicative of fluid temperatures
<180˚C. These barren systems do not show evidence of boiling related quartz textures
and contain only trace amounts of pyrite ± chalcopyrite and no electrum. The
Chapter 6 Conclusions
107 Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
characteristics of a deep level formation in gold bearing systems at Wang Yai are
consistent with current epithermal models. At deeper levels gold bearing vein systems
such as Conical Hill, T1 Hill, T4 Hill, S.V and A,F-(Gift) are characterized by the
predominance of massive crystalline quartz, crustiform colloform banding, replacement
textures and precious metals (argentite and electrum). The predominance of crystalline
quartz in these systems suggests higher temperatures of formation than barren systems.
A higher formation temperature for these systems can be explained by formation at
deeper levels in the system. The quartz textures and mineralogy in these systems suggest
a level of formation at or above the zone of boiling and precious metal deposition.
Quartz replacement textures such as such as silica pseudomorphs after adularia and
bladed quartz suggests ore deposition may have been in response to extensive boiling.
The occurrence of precious metals and negligible base metals is consistent with the zone
of extensive boiling.
6.3 Fluid characteristics
Oxygen isotope studies of vein quartz have revealed a correlation with respect to quartz
textures and gold grade. Low � O18 values are associated with high gold grade and
crystalline quartz dominated vein textures. High � O18 values are associated with low
gold grade and chalcedony/opaline silica dominated vein textures.
Based on the relationship between homogenisation temperatures and mineralised veins
fluids responsible for ore deposition are interpreted to be in the range of 200 – 250ºC.
The high variability in salinity values could indicate that extensive boiling occurred
during mineralisation at Wang Yai.
Sulphur isotope values from gold bearing veins indicate the source of sulphur may have
come from depth or from volcanic host rocks, through which hydrothermal fluids passed.
Chapter 6 Conclusions
108 Geological setting, nature of mineralization, and fluid characteristics of the Wang Yai prospects, central Thailand
Significant findings and implications for exploration.
• Hand specimen analysis of quartz textures and gangue mineralogy in vein
samples can be used to delineate vein systems that are favourable hosts of gold
mineralisation.
• The relatively shallow formation depth for vein systems at Wang Yai suggests
that there is great potential for mineralisation at depth.
• Vein systems that should be given the highest priority for further exploration
include Conical Hill, Conical west, T4 Hill, S.V, and A and F system in the Gift
prospect.
• Barren vein systems indicative of shallow formation should not be disregarded as
potential for mineralisation may lie at depth.
• Oxygen isotopes values of vein quartz show a strong correlation with gold grade
and gold ‘kind’ quartz textures which implicates their great potential for use as a
vector to ore.
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